Sabotage the ADVENT Monument
If you don't have the Tutorial enabled, this is your first mission of the game, always given the codename 'Gatecrasher'.
It has the unique distinction of being a mission that uses the City Center plot type without being a VIP mission. Amusingly, this extends to it always having fully functional security towers placed on the map, with random hack rewards and everything, even though it's impossible to Hack them without mods of some kind.
The reason it's impossible being of course that you have a predictable, out-of-your-control initial squad setup that cannot include a Specialist or SPARK. The exact setup depends on two things: are you playing War of the Chosen, and are you playing with Lost And Abandoned enabled? If neither of these things are true, or if both of these things are true, you will start with four Rookies equipped with of course Kevlar Armor, a Conventional Assault Rifle, and carrying a Frag Grenade apiece. If you're playing War of the Chosen without Lost And Abandoned enabled, one of those Rookies will be subbed out for a Squaddie Reaper, Skirmisher, or Templar, depending on which Resistance faction you're starting the run in contact with. They of course are all equipped with their Conventional-tier gear, and aside the Reaper (Who has no starting Item slot) they carry a Frag Grenade just like your Rookies.
The actual mission itself is, oddly, a pretty rough introduction to the game that isn't really representative of the difficulty of the very early game. Below Legend, you'll get six enemies to face, which is pretty normal for the very early game, but in regular early missions they'll be broken up into three pods of two units: in Gatecrasher, they're instead two pods of three units, specifically a pod of three Troopers and a pod of one Officer and two Troopers. This makes it impossible to pick off enemies two at a time without a chance for enemies to fight back, which is pretty safe for even Rookies to try to do in early missions.
Legend adding in a third pod that's one Sectoid leading one ADVENT Trooper is less out of line with its early game, at least.
It does at least mean this is a great mission to go for an Overwatch ambush, much more so than in a regular early mission: you get to catch three enemies instead of two, the City Center plot type means you can usually get high ground via climbing atop a building, and a bunch of Rookies don't have access to a pile of abilities that don't play nice with Overwatch ambushing. Below Commander, such a high ground Overwatch is extremely likely to kill every enemy: if the pod isn't near Cover, you'll get four 85% accuracy shots that one-shot these Basic ADVENT Troopers, meaning missing once is fine. You'd have to miss twice for there to be survivors, which is pretty unlikely.
Alternatively, you can skip all that nonsense and just throw a Frag Grenade at the first pod to instantly delete them. If they happen to be carrying timed loot -and somebody is carrying timed loot on this mission, period- you'll be blowing that up, but that's not too big a deal if you just want to be done with Gatecrasher already. Given it's actually one of the hardest missions in the first couple months, that's a fair position to take.
If you're above Veteran, things get quite a bit dicier. That Frag Grenade has decent odds of deleting one Trooper (Not far off from a 50% chance), but is ludicrously unlikely to delete all three, and even two is very low odds. Similarly, the high ground Overwatch ambush can not only miss, but can low roll on damage to leave a Trooper intact. It's thus much more likely you'll have a Trooper survive to fight back -one miss and one low roll immediately produces a survivor and isn't terribly improbable- and if they get a crit then one of your poor Rookies is already dead. (Or maybe Bleeding Out, but you don't have a Medikit to save them with, so...)
As all your high ground is invariably Low Cover, the odds of being hit back are pretty solid, too, if you arranged this high ground Overwatch ambush.
And then you have to go take on another, stronger pod without the benefit of Concealment and possibly down a soldier! Two pods on Legend!
Indeed, up on Commander and Legend this mission is one of the most luck-based-out-of-the-player's-hands missions in the entire game. It's nearly impossible to get perfect accuracy shots with a Rookie (They need high ground and to be close to the target and to have no Cover interfering), you absolutely do not have enough Frag Grenades to skip the RNG even if we make the ridiculous assumption that each pod conveniently clusters for every grenade toss (The Officer would still survive with 1 HP in this wholly imaginary scenario, and on Legend the Sectoid and their friend would be a completely untouched 14 HP to chew through), and you don't have any tools for mitigating enemy accuracy or the like. If you play enough runs with the Tutorial disabled (And remember: you can't enable it on Legend) you will find yourself failing this mission through nothing more than the RNG deciding to be moderately hostile.
War of the Chosen giving you a Resistance soldier to start noticeably helps in this regard: a Templar is a no-questions-asked deletion of a Trooper per turn and can follow up a Frag Grenade on the Officer or Legend Sectoid for an assured kill once they have some Focus (1 Focus will ensure the Officer dies, 2 Focus will ensure the Legend Sectoid dies), and less dramatically a starting Reaper or Skirmisher is still a pretty big buff to the squad's effectiveness.
The Reaper can, if patrol behavior cooperates, completely delete the Trooper pod without even breaking squad Concealment: get close, start a turn by tossing the Claymore, then have the Reaper shoot it. Now you're free to find and Overwatch ambush the Officer instead of having them run to High Cover the second your squad is close. Or find and Overwatch ambush the Sectoid's pod if you're on Legend and more scared of it, though I'm not sure why you'd be.
The Skirmisher's Justice is a very reliable deletion of a Trooper (If it grabs them, they die), and can make the Officer much less painful: if they grab the Officer as their first action, the Skirmisher can follow up with a 100%-reliable shot that will kill them in conjunction with the Ripjack hit. If Justice misses, it has pretty good odds of smashing the Officer's Cover, which tilts the odds a lot. Less obvious is that the built in Grapple lets the Skirmisher achieve otherwise-impossible flanks the AI doesn't account for at all. Thus, in spite of being hampered at longer ranges by the Bullpup being Short-ranged, the Skirmisher will tend to drag up the squad's performance in an RNG-resistant way.
Notably, the Templar and Skirmisher also bring in a crucial extra hit point to the squad: it's far less likely for them to end up abruptly dead than one of your Rookies, in turn improving the odds you squeak through the mission even if the RNG proves hostile.
On the plus side, the game is kind enough to give you an arbitrary Game Over and ask if you want to retry the mission in the event your squad is wiped, and in fact the pause menu will let you restart the mission anytime you like. This does not generate a new version of the mission: you'll get the exact same map, exact same enemy positions, and for that matter exact same squad. So if the reason things went wrong is because the Officer pod was closer to the Trooper pod than you'd expected and you pulled it when you weren't done with the Trooper pod, you can avoid repeating that mistake.
I imagine the real reason for this Game Over is plot and Research tree stuff, mind, as the game needs you to immediately have an Officer corpse, and if you don't have the Tutorial enabled this mission is how the game foists the corpse on you: this is a mission in which you collect bodies (Which is narratively very bizarre, but shhh), and also a mission in which the objective demands you kill everything on the map. In conjunction with the rigid enemy composition, succeeding at this mission by definition means you have an Officer corpse.
Still, it's nice; I imagine if the game didn't Game Over you here with a restart offer many players would still just start over the campaign rather than accept a failure on literally the first mission of their campaign.
It's also interesting that Legend difficulty thus gets an assured Sectoid corpse to start the game with. Not terribly meaningful given you won't be able to Autopsy it until your first Scientist is acquired and this will normally happen after a Guerrilla Op has already given you a Sectoid corpse, but still a little interesting.
Anyway, the mission, in addition to requiring you to kill everything on the map, also requires you set an X4 charge at a specific point on the map. This is handled in a mildly inane way, as you still have to manually do it once every enemy is dead, never mind that no time pressure exists and all: it basically just means some portion of the time you'll have a soldier finish Bleeding Out or have timed loot run out after you killed everything but before you got the X4 charge set, which is annoying but not terribly meaningful. I'm assuming it's meant to ensure new players get introed to objectives needing to be done in addition to killing everything else, if they skipped the Tutorial?
In any event, the X4 charge target comes in one form in the base game and two possible forms in War of the Chosen: in the base game, you're always blowing up one of those artsy statues of an Ethereal, whereas in War of the Chosen it can be...


... with the first cinematic depicting him as part of an operation, accepting a bit of violence from an ADVENT Officer, and then shrugging it off and ordering the mission to go. And of course the Tutorial then has him show up for Tutorial Gatecrasher, with more cinematic moments of participating in combat (Shooting the last of the initial Troopers before he joins you as a gameplay piece), and then being an actual in-engine combat unit.
Past this the game doesn't allow itself to put Bradford in danger (Yes, I know, Alien Hunters, I'm getting to that), but there's still things like Bradford getting (understandably) yell-y in the lead-up to the Avenger Defense, contrasting with how EU/EW Bradford was perpetually soft-spoken.
If I'm correct, this would all be kind of... lame, honestly, looking more like insecurity to me than like a desire to make Bradford more distinct of a character or the like.
But, the interesting thing is now we get into how Bradford's actor plays a lot of Bradford's lines, and the trend there; in many cases where there's no obvious visual cues (eg Avenger dialogue, where your support staff are just 'talking heads') Bradford sounds tired. There's one particularly good example late in a run where Bradford muses to himself about how the aliens 'have always been monsters', where he talks about the constant Ethereal science experiments and all, and he sounds more exhausted and worn down than, say, outraged.
I strongly suspect this is something the actor brought in organically, rather than because it was the intended direction to take Bradford. It seems a bit dissonant with the attempts to illustrate Bradford's 'badassery', and I personally am familiar with Bradford's voice actor from The Darkness 2, where he plays Jackie Estacado (The player character) and once again some of the strongest acting centers around the moments where Jackie is depressed and exhausted.
And the thing is XCOM 2 seems to have noticed this and responded to it.
Alien Hunters is already a bit of a tilt in the course; while the gameplay endeavors to make Bradford seem cool and powerful and all, there's no cinematic beats of Bradford doing something violent, and Bradford's dialogue is a lot more focused on things like hoping Vahlen is okay than it is on getting vengeance or the like. War of the Chosen's new dialogue is also in line with this tilt -it's worth pointing out that the new start-up cinema has Elena saying "Reapers are always sure" which is the kind of dialogue I'm used to seeing from stories trying to make a character seem cool and all, and Bradford just kind of refuses to engage with Elena being weird. (It's just too easy to imagine him rolling his eyes as he says, "I'll take your word for it," though of course we don't see such since we're watching Elena) Which is to say War of the Chosen places Bradford in some sense in opposition to the 'cool badass' style, instead of continuing to push Bradford into that framework..
But what really stands out is the Tactical Legacy Pack, which is about 80% Bradford being a Tired Old Man reminiscing about Days Long Gone, while of course depicting stuff that happened when he was younger than he is at the start of XCOM 2. It's a very clear embracing of this Old Tired Soldier Bradford.
And I really like this!

There's a couple reasons why. Part of this is the relatively personal; I just think Tired Old Man Bradford is a more interesting and appropriate Bradford than Cool Grizzled Badass Bradford. XCOM 2 is a sequel where X-COM lost and Bradford has been fighting ineffectually for twenty years, with most of the people he knew dead or at least missing. That's the kind of thing that wears people down hard, and honestly if XCOM 2 had stuck to trying to push Cool Bradford it's very possible it would've hearkened back to the EU/EW character problems; someone who is completely unaffected by a total defeat that kills a lot of people they know is going to come across less like a Cool Badass and a lot more like they just didn't care about any of those people, which starts raising concerning ideas like 'maybe he'd happily send current X-COM troops to their deaths absolutely gratuitously, too'. Old Tired Bradford is much more believable characterization, and avoids really unfortunate undertones to boot.
The much bigger reason why I like it has to do with broader institutional knowledge considerations; that there are a distressing number of games that get made where they very obviously are wanting to make themselves more like movies... while failing to learn from some of the obvious lessons learned in Hollywood and so on.
The pertinent one here is the consideration of how an actor interprets and depicts a character when it comes to the stuff people often don't consciously think about; tone, emphasis, body language, and all the other little things people react to without necessarily realizing it. Word choice -the stuff a script obviously dictates- matters, but how things get said matters too, often a lot more than word choice.
TV series, movies, and so on recognize this in several ways, such as typecasting, where someone proves they sell a particular kind of character well and so people are quick to cast them in a similar role, but the piece I'm focusing on right now is the feedback loop that even the audience can see relatively readily in longer-running TV series and cartoons, where early episodes are often subtly off compared to later episodes as everyone involved gets a handle on the important parts of the series and on the exact way characters get played by actors. (Among other points, how actors play off each other) In some cases this is quite dramatic, where it's very visible to the audience that a character was imagined one way in initial writing, got played in a very different way, and then later on the character's underlying writing shifts to be in line with the way their actor played them...
... which is exactly what it looks like happened with Bradford.

Furthermore, while Bradford is the most dramatic example, I have a similar impression of Shen and Tygan starting out from a subtly different concept and then later content converging on the way their actors play them.
My first impression of Shen was, similar to the handling of Bradford, that she was meant to be something like a more 'hardcore' version of her father, someone who has been hardened by 20 years of ADVENT oppression, the loss of her father, etc. The base game has her suspicious of Tygan for being ex-ADVENT on and off in dialogue, her design includes a tattoo marking her dedication to fighting the oppressors, and while she's not as bluntly lusting for blood as base-game Bradford she's still more clearly hostile than your support staff were in EU/EW. (Where they were dedicated to their job of fighting the aliens but didn't seem to hate the aliens for the things they were doing or the like -they were mystified by stuff like Terror missions, not enraged and wanting revenge or the like)
There's some bits like how she treats ROV-R like a pet that could be argued as running contrary to all that, admittedly, but I'm giving context here to note that Shen's actress doesn't play Shen anywhere near as angry or bitter or the like as I might've expected if that was indeed the original thought process. Shen ends up seeming more fundamentally optimistic than your other crew, more motivated by moral outrage, and, interestingly, ends up with her skillset feeling pretty secondary to her personality, which stands out to me when her skillset clearly drove decisions like where she speaks up at all. (eg she has things to say about robot enemies, where non-robot enemies tend to get Bradford and/or Tygan talking)
And then, like Bradford, later writing sticks closer to Party Conscience Shen than Hardcore Chief Engineer Shen, with Shen's Last Gift being a story centered on familial connections and War of the Chosen's added dialogue largely staying in that range. Also like Bradford, it ends up working better than what I suspect was the original plan, if a bit less dramatically.
(The funny thing is I'd probably have disliked it if the Shen we got had been the dev's starting point for writing her, as it would've come across like We Made The Female Character Stereotypically Feminine Because Girl When That Doesn't Make Sense Here... but because it happened organically, I'm praising it instead)

With Tygan, meanwhile, my first impression was that he was just Vahlen all over again in terms of personality, and... in some sense I think that's still accurate to the final result, actually, in that the broad strokes of being a Scientist Character who is fascinated by science and tends to focus on it over considerations like ethics, but the specific vibe manages to set them apart anyway.
With Vahlen, the Science Over Ethics mentality always came across like sloppy writing rather than deliberate characterization. When Horrifying Things are happening in EU/EW, Vahlen tends to be openly appalled; it's scenes like 'hey Bradford, can you please put soldiers in serious danger so we can satisfy my curiosity about live aliens?' that end up with the undertone of Vahlen caring far more about her scientific curiosity than things like human welfare, and it looks more like the devs didn't really think too hard about how the conversation comes across (Since its primary purpose is to lead into the Arc Thrower, an important gameplay mechanic) rather than a deliberate attempt to write Vahlen as being pretty uncaring or the like.
With Tygan, the acting ends up painting a picture of a man who isn't readily moved to emotional displays, doesn't experience gut-level horror very readily, has a strong fundamental interest in science for its own sake (vs Vahlen attaching a certain amount of moral imperatives to science; there's a sense she thinks that science is worth pursuing because it's a moral good rather than out of a personal passion), and ethical concerns not as a gut-level horror thing but rather as a more abstract set of guidelines with conscious thought underlying them. Honestly, he ends up coming across pretty strongly autistic, which is interesting given I doubt anybody involved sat down and went 'autistic Tygan, let's go'.
This ends up hitting the same broad notes as Vahlen, but with a clear, coherent personality underpinning things like focusing on Science Things while everybody else is reacting to something else entirely, so the final result still feels very different from Vahlen in practice, even though it's effortless to construct a summary that describes both of them equally well.
Notably, Tygan being ex-ADVENT ends up coming across differently than I suspect was the core concept; it's easy to imagine the Tygan we got working under ADVENT, catching glimpses of less-than-great elements of ADVENT but not having an obvious reaction to such, where he got trusted with relatively important work because he seemed like someone genuinely unbothered by ADVENT's atrocious ethics... while in actuality he was quietly keeping an eye out for an opportunity to get out, because he absolutely decided This Is Wrong but was able to keep his opinions close to the chest the way a more openly emotional individual would likely fail to manage. That works really well, and I'm pretty sure wasn't actually what the devs had in mind to start but rather is an undertone the actor brought in organically.
Tygan doesn't get as much further development as Bradford and Shen do -the closest he comes to having a DLC mission to himself is being a VIP you escort in a Tactical Legacy Pack mission, and he doesn't even get dialogue there- but it's still the case that later content sticks more firmly to Tygan The Low-Affect Scientist than the base game, which has a few moments that seem premised on the idea of Tygan being more openly emotional than what we got, and a number of other moments that work very well as-is but where it's still easy to imagine the writing intent was for Tygan's response to be more emotional than what was acted. (eg at the Blacksite, he talks about, "so many victims... processed with such brutal efficiency,"
which is a line that could so easily have been played as sick and angry horror, as opposed to his actual, rather understated response)
It's neat to see a video game actually reacting to and building on what the voice actors brought in, and potentially promising for future games getting stronger characterization going. Certainly, Chimera Squad does a pretty good job of having your agents fit to their voice acting, much more so than I'd have guessed before playing it.
Anyway, moving on to...

Recover ADVENT Power Converter
... mission the second unique to Tutorial-land!
Mechanics first, though, as this is a Real Mission, and in fact effectively takes the place of Gatecrasher even though the Gatecrasher codename gets used for the prior mission. After all, you don't loot anything in the Tutorial mission, and in fact the only mechanical benefit you derive from it is that Jane Kelly will promote into a Ranger. This does mean that in Tutorial-land your First Real Mission is three Rookies and a Ranger instead of the usual four Rookies, but the point is that this mission is the one where you get your Officer corpse and all.
Unsurprisingly, it thus shares with (non-Tutorial) Gatecrasher that failing the mission results in a Game Over and the game prompting you to restart the mission. Also like Gatecrasher, it's a bit of a rude introduction to Real Gameplay, and in fact is ruder than Gatecrasher!
First of all, this mission is timed, with 8 turns on the clock to start. Your objective is to...
... Hack this device before time runs out. Reminder: you cannot have a Specialist or SPARK for this mission, so you will be forced to walk somebody adjacent to the Power Converter to Hack it, making the timer tighter than it looks if compared to a Guerilla Op with a similar setup. For that matter, compared to Gatecrasher you're dealing with needing to spend an action point on the objective -if Gatecrasher did have a timer of equal length, it would still be easier as a result.
Second, the mission has a much more menacing enemy set than sub-Legend Gatecrasher, having three pods, where two of them are an Officer and a Trooper apiece and the third is a Sectoid and two Troopers, for 1 more total enemy than a below-Legend Gatecrasher and 33% more enemy HP to chew through on Commander! You do have a guaranteed Ranger to try to pick on the Sectoid's weakness to melee, at least... but for one thing the emphasis there is on 'try', since she won't have Blademaster to reach 100% accuracy. And if she misses, it's dangerously likely the Sectoid will crit her and kill her as a result.
This rude intro is particularly egregious in War of the Chosen, where if you don't enable Lost And Abandoned you'll start with a Squaddie Resistance faction soldier, but even in the base game this is a pretty noticeable spike in difficulty compared to the non-Tutorial mission it's the meaningful counterpart to. (Non-Tutorial Gatecrasher) This is actually something of a trend with the Tutorial in general, honestly, where it actually makes the very beginning of the game overall harder than normal, which is particularly striking given the early game tends to be pretty rough already due to your limited options and so on. It's something of a subtle mercy that Legend doesn't let you inflict the Tutorial on yourself, honestly, and if it weren't for how Rookie and Veteran are so effortless I'd be recommending against turning the Tutorial on at all -as is, I recommend you do it once on Rookie or Veteran difficulty and never again. (Unless you don't mind the challenge and/or are wanting to rewatch the Tutorial-exclusive cinema bits, anyway)
Among other points, this rudeness makes it a lot more likely you'll straight-up lose people, putting you on the back foot in the early missions, which can cascade into more difficulties down the line where you lose someone in your third real mission because you lost a couple people in your second real mission because you lost a couple people here, etc.
Also, it's worth noting the game will actually let you mess with soldier loadouts, but the Tutorial rails mean this basically doesn't matter in the base game (You can have Jane Kelly wield a Rifle instead of a Shotgun, but why would you?), and only slightly matters in War of the Chosen if you have the Tactical Legacy Pack. (As you'll be able to equip people with the TLP weapons, and should do so if you've unlocked them) The game doesn't let you actually go to Engineering until after this mission, after all.
Anyway, the mission itself is a procedurally-generated mission using the Wilderness plot type (I have yet to see it get the Xenoform biome in War of the Chosen, but I haven't done enough runs with it enabled to be confident it's actually barred) that can be loosely compared to the convoy Supply Raid variant in design; you're in wilderness with a central lane of ADVENT vehicles, you start with squad Concealment, you'll loot bodies... except of course that such a Supply Raid doesn't have a Hack objective nor a timer. It's straightforward of a mission, but still pretty rude, especially since the game uses it to try to introduce you to Overwatch ambushes while not being a mission that's particularly friendly to leveraging them -no well-placed high ground, for one.
I'm honestly kind of curious why the Tutorial has these tuning oddities. I've seen plenty of problems across plenty of video game tutorials, but I don't think I've ever seen a tutorial so clearly make the game harder for the player. I've seen plenty of tutorials that force the player to perform sub-optimal actions, either because they want to introduce a concept now even though it's not really something the player should mess with until much later or because the devs honestly think they're teaching you good play while their game incentivizes different behaviors than they think, but for one thing these are usually failings that only hurt in the long term. 'Turning on the tutorial actively makes early missions greater challenges' is a new one on me. Was the game originally tuned to have a tougher early game, got rescaled to more or less what we see now, and the Tutorial wasn't brought in line with the new tuning? It's weird.

Narratively, this mission is odd, but not obviously odd, requiring a bit of thinking to become confusing.
The essential concept is that the Avenger can't fly yet, and you're here to grab a power core widget so Shen can get it installed and the Avenger can fly. Also, it's unstable, which Bradford blames on fighting that's already occurred before you arrive, which is why you need to Hack it, because it will blow up if you don't. Okay, sure.
Except, wait, if it's prepping to blow, why is ADVENT just standing around, doing nothing about it? Shouldn't they have their own tech person working to fix it, one who actually knows what they're doing rather than a random X-COM yahoo who probably has zero familiarity with this device in particular? (Reminder: much of the game emphasizes that X-COM the organization is completely ignorant of much alien stuff, making this even more eyebrow-raising) Or alternatively, if they don't expect to be able to fix it, shouldn't they be fleeing the blast zone instead of diligently patrolling for rebels?
It's also somewhat weird that ADVENT is conveniently carting around a plug-and-play piece of UFO technology that is, in fact, compatible with the Avenger, which is ostensibly a converted Supply Barge, given there's nothing to suggest planet-side Supply Barges are a thing (As opposed to, say, Battleships, which are never seen by the player but do get name-dropped in the Avenger Defense), UFOs in general don't seem to be a major part of ADVENT's operations, and a UFO part being carted around on a ground convoy is not intuitive for that matter. This piece isn't untenably strange, with it possible to justify each individual oddity without distorting what the game does tell/show of the world (eg it's plausible UFO technology is designed to be sufficiently modular that there's parts shared between Supply Barges and Battleships, including this device), but it's something that could've been more clearly contextualized.
More subtly, this whole scenario doesn't really jive with the rest of the game, where the Avenger's status as an air-mobile HQ is not only true but is presented as a long-standing thing. You get jokes about Bradford being terrible at piloting the Avenger, but they're not presented as 'Bradford is still learning to fly this thing', even though it would be effortless to do so, as one of the most direct examples.
I suspect this whole thing was driven by a combination of 'cool factor'...

... given it lets the game do this visually striking shot of the Avenger first taking off...
... and the devs basically casting about for a low-consequence early mission for the Tutorial, equivalent to Gatecrasher ('Low-consequence' in the sense that it doesn't advance the main plot, primarily), and arriving at 'the player has to work to arrive at a normal game start state' as their answer. Probably as a somewhat thoughtless implementation of industry norms, honestly: there are so many games with an optional tutorial where the tutorial has plot-y bits and skipping the tutorial just takes you right past those early bits but still treats them as more or less canon.
Whatever the reasons for arriving at this point, I tend to discount this mission from a narrative perspective, as it fits so poorly into the rest of the narrative and isn't important to the rest of the story. (With non-Tutorial runs making this very obvious by skipping it with no acknowledgement)

A few minor oddities before moving on.
Firstly, the icon I've used for this mission actually is it's real icon, but you'll only see it during the Skyranger loading screen, as the game forces you into this mission without ever interacting with the Geoscape. This is a unique distinction, out of missions that have custom mission icons like this.
Second, this mission has the Tutorial once again present a misleading picture of core mechanics, in that you're forced to start the Research on the Commander's chip ('Alien Biotech') before starting this mission and then it's finished when you come back from the mission -which can't happen in real play, or at least requires really bizarre edge cases to happen.
Third, a really odd, easily-overlooked change in the narrative is what happens when you're first being introduced to the Avenger's bridge area: this has slightly different cinemas for a Tutorial run and a non-Tutorial run, with the primary difference being that some manner of security measure doesn't like the Commander in a Tutorial run, requiring Bradford to overrule it, with this stuff not happening in a non-Tutorial run. It's weird that this difference exists and I'm intensely curious what the story behind this particular difference is.
Fourth, enabling the Tutorial actually has the Avenger start out with no cleared rooms. After this mission, you get railroaded into your standard first Guerrilla Op -the one that always gives you an Engineer and isn't connected to a Dark Event and so on- and get tutorialized on Excavation mechanics. It's a bit odd, and has me wondering whether that's just a fundamental choice to have the Tutorial be weird on this topic (To justify you not interacting with Excavation and construction until the Tutorial wants to instruct you) or if at some point the Avenger having a pre-cleared room wasn't the default start state.
The Tutorial has so much weirdness attached to it...

Sabotage The Alien Facility (Avatar Project Facility)
The only mission in this post you're liable to see more than once in a run, and one of only two in this post you can see more than once in a run. The base game genuinely leans pretty hard into its standard mission pools -War of the Chosen, as we'll see next post, is a lot more fond of oddball missions that don't fit into a broader pool but aren't once-per-run either.
Sabotage The Alien Facility itself is, of course, tied to Avatar Project Facilities, requiring you either get in contact with a region containing an Avatar Project Facility or perform a Facility Lead Research to unlock access to a specific, randomly-chosen Facility... which itself requires you have a Facility Lead: you can get one as part of a timed loot drop, as a reward for a Hack, by buying one from the Black Market, or in War of the Chosen by performing a specific Covert Op. All of these Facility Lead options occur fairly rarely; it's not unusual for a run to reach the end and have never acquired a Facility Lead at any point. So you should largely be planning around contacting their regions; don't plan around Facility Leads unless you've already gotten at least one.
I've already touched on this some before, but to reiterate...
Avatar Project Facility generation occurs roughly once a month or so unless you get the Dark Event that hurries the next one's construction. Where they're placed is difficulty-dependent: your first one is always placed 2 Contacts out unless you're on Commander in which case that first one is 3 Contacts out, while further ones will be placed 2 Contacts out, 1-6 Contacts out, or 2-6 Contacts out, depending on your difficulty. (Rookie/Veteran for 2, Commander for 1-6, Legend for 2-6) 'Contacts out' is as in the minimum number of Contacts you'd have to spend to reach the Facility. Also, there's actually a hard limit of 5 such Facilities on the map at any given moment, though I'd honestly be a little surprised if someone had actually reached this limit in an unmodded game; realistically, you're going to hit a Game Over before that has a chance to happen. (I suppose this cap could come up more readily with Lengthy Scheme enabled...)
Completing this mission will of course remove the targeted Facility from the map, remove whatever pips of Avatar Project bar progress are attached to the Facility, and less obviously it will actually temporarily stop further Avatar Project bar progress from occurring. This last effect is a fixed, non-stacking duration: it's better to hit a Facility, wait a week or so, then hit another one, than it is to hit both in rapid succession, as far as this goes. These are the entirety of the benefits of completing the mission: it has no external reward and you don't loot bodies.
Integrated DLC in War of the Chosen adjusts this statement slightly since you'll need to be hitting Facilities if you want the Alien Ruler Armors, but there's still no other direct payouts for completing a Sabotage The Alien Facility mission.
Tactically, Avatar Project Facilities always use the Wilderness plot type, all biomes possible. The actual map is usually primarily wilderness, with a single large complex that is of course your actual objective. Sometimes there's roads up to the building and attendant vehicles on them, but quite often the building is 'freestanding', with no evidence of a way to readily reach the facility. (One assumes ADVENT relies on their flying transports in those cases) The building itself can have Turrets mounted across the roof, and usually has at least one or two.
Completion of the mission, it should be noted, does not require killing every enemy on the map. It's not realistic to stealth this mission, not usually, but it's perfectly okay to simply bail once you've set the X4 charge, such as because your squad is in bad shape and you're not confident in their ability to take on more enemies. In War of the Chosen, with the Fatigue system, there's a decent argument for doing so fairly consistently to maximize the odds the soldiers are available for more important missions.
Anyway, the actual objective is to set an X4 charge at a specific point inside the building. (This point is always adjacent to a completely indestructible pillar, ensuring your soldier has something to set the charge on) Just as in Gatecrasher, this is a free action, albeit one that requires the soldier has at least one action point. Notably, setting an X4 charge always breaks Concealment -this is part of why stealthing these missions isn't particularly realistic. Exacerbating this point is that the pillar is indoors, and you can only set Evac points in locations with significant access to the sky; it's often possible to just set the charge, set an Evac zone inside the soldier's Dash radius, and run, but some layouts make this infeasible, especially since there's always a pod assigned to guard the X4 point.
Once the X4 charge is set, you just need to set the Evac and get the squad out to complete the mission. You'll get to watch a little cinema of the Facility blowing up, and the mission is done.
This mission kind of makes me wish Evac mechanics weren't so thoroughly convenient. The ability to call an Evac where you need it the second you need it with no delay or anything is usually a low-key way to be friendly to learning players, where a mission can go horribly wrong and they reduce the final damage by bailing as soon as it's obvious to them that the situation is beyond their ability to salvage, but in this particular case it sucks out a potential bit of challenge from the mission. Among other points, setting the X4 charge actually triggers a reinforcement flare to drop immediately -even into War of the Chosen- but once you understand the relevant mechanics it's completely trivial to set up to Evac before the enemy turn and thus before the reinforcements have a chance to arrive.
Just something like a two-turn delay before the Evac zone was ready would make this a notably more interesting mission. It feels like a missed opportunity. (Though admittedly it would be aggravating to have the delay in basically every other mission it could apply to)
Narratively, this is straightforward enough if you accept that a decent amount of abstraction is going on. That is, there's no scientists or the like on the grounds, but obviously somebody has to be doing research in these buildings, so their failure to be represented is because they wouldn't be mechanically relevant (Or would complicate the design with limited gameplay payoff), not because they're not supposed to exist.
Similarly, there's a fair case to be made that Avatar Project Facilities realistically ought to provide clues to the nature of the Avatar Project, but they're fundamentally designed to serve the gameplay as a source of Avatar Project bar progress that can spawn and then be smashed by the player. It would be problematic to actually tie up plot progress stuff into them, one way or another. So even though it doesn't make realistic sense... whatever, it's serving a function, and it serves that function well enough.

Avenger Defense, UFO edition
This could be argued as a 'golden path' mission in that making progress down the 'golden path' will eventually trigger a UFO hunt and the first UFO hunt always succeeds in triggering this mission; in conjunction with failing this mission being a Game Over, I'm pretty sure it's impossible to complete the game without succeeding at this mission, much like the plot missions. I'm putting it into 'miscellaneous' because it doesn't progress the plot, or unlock a Shadow Project, or otherwise behave like the 'golden path' missions.
Anyway, I've covered how this happens before, but to reiterate: each time you make progress on the 'golden path' (Complete or unlock a Shadow Project, basically) there is a chance a UFO will generate and Bradford will instruct you to fly carefully. Once again, this is a lie, and the UFO will find you no matter what after a partially-randomized amount of time, completely unaffected by your flight pathing. If this is the first time in a given run that a UFO has intercepted the Avenger, then this mission is guaranteed to kick off. If it's a later time, it will either automatically be evaded (Rookie/Veteran) or have only a chance of kicking off this mission. (25% for Commander, 50% for Legend)
Completing the mission itself offers no reward beyond, you know, avoiding a game over. No Supplies or Intel or whatever, and you don't loot bodies, and in fact enemies will refuse to drop even timed loot in this mission! So don't be thinking it'll be a big payday mission.
A unique distinction:
It has its own loading screen, seen no other time.
Technically this behavior is not unique, as Lost Towers also has a personal loading screen used when transitioning between maps, but the overwhelming majority of missions use one of three approaches: the standard Skyranger loading screen, the Propaganda Images loading screen set used primarily for first loading up a file, or hiding that any loading is happening through cinemas. (The Alien Fortress does this last one, for example) It's a little interesting to me that this mission is an exception, and indeed is the only exception if we discount DLC missions.
Tactically, this mission uses the Wilderness plot type, with all biomes possible. Like most Wilderness missions it's actually pretty rare for it to be just woodlands -there's usually old buildings on the map, and a barn is often adjacent to the mission objective- but it does sometimes happen.
This mission also has the unusual distinction of allowing Berserkers and Chryssalids to spawn while not being a Retaliation mission. (No Faceless, though) Note that if Chryssalids spawn in, none of them will be Burrow loner-pods, which makes realistic sense (ADVENT is on the attack, not the defense, with a clear target to pursue) and is honestly just nice from a gameplay standpoint given how frustrating that would be in this mission.
The Chosen, meanwhile, are forbidden from spawning into this form of Avenger Defense. I've similarly never seen ADVENT Turrets on this mission and would honestly be surprised if they're at all possible.
Alien Rulers, however, are absolutely allowed. You're not particularly likely to see one in this mission, especially in the base game where odds are good you'll kill them all before this mission gets around to triggering, but it is possible. So that's something to be ready for if you've got an Alien Ruler on the loose and a UFO hunting for the Avenger.

On of the topic of enemies, an unusual element of this mission is the primary way to fail it is to let an enemy get aboard the Avenger via the ramp your squad starts in front of. (And is conceptually using to exit the Avenger) This is an instant game over if even one enemy manages this, but thankfully it's less dire than you might intuit -an enemy ending their turn in the area is not an immediate game over. So long as you're not ridiculously sloppy, you can almost certainly avoid the game over by killing them before their next turn rolls around.
Naturally, the enemy AI does actually prioritize getting to the ramp area in this mission. The actual execution is a bit clunky, unfortunately; each enemy quite clearly has a chance of deciding that this turn it's making a run for the ramp, in which case they will simply Dash to the ramp area. This can lead to really obviously sub-optimal behavior, like a Berserker failing to attack anyone even though it absolutely could've ended its turn in the ramp area by attacking someone in it. Notably, this decision can kick in on any currently-active enemy, even if they're nowhere near the Avenger, where it's almost certainly a waste of their turn. This behavior also actually overrules the AI's normal preference of trying to avoid walking into clouds and fires, as another way the implementation is janky.
Even if you do a poor job of preventing enemies from running past your squad, Bradford sends one of your not-yet-deployed soldiers every second turn, with them appearing right in the ramp area. So if you do let an enemy slip past where your deployed soldiers can't catch up, you're still going to have someone pop out, ready to handle the defense. It's actually pretty hard to lose this mission.
The Avenger ramp area is fixed, unsurprisingly, offering stretches of Low Cover all along the edges of the Official Ramp Zone, punctuated by a couple stretches of High Cover in the front, and with a second layer of Low Cover stretches a bit further out. The rest of the map is much more variable, surprisingly so given the once-per-run missions have large stretches of fixed terrain and larger stretches of terrain that's not fixed but still has less variability than usual. Avenger Defense is technically not once-per-run, but it's very close given that a second UFO can only generate if a specific Dark Event generates, goes through, and then has its interception succeed; in over two dozen full runs, I've yet to actually perform this mission more than once in a given run. It's a bit odd.
Anyway, there's usually some high ground near the Avenger's ramp, though position and nature is quite variable and occasionally it doesn't happen at all; sometimes you'll get a partially collapsed building dominating the area right in front of the ramp, sometimes you'll get a power station where the building part is actually pretty far from the ramp, sometimes you'll get a haphazard collection of natural cliff terrain in front of the ramp... I've probably not seen the full range, honestly. Still, it's a good idea to keep an eye out for high ground, especially since this is one of the missions Sharpshooters get to shine in -they can fire on the spike reliably no matter how far, and this mission's unique dynamic is quite favorable to setting them in a sniper nest near the ramp for most of the mission.
Past that usually-but-not-always high ground, there's usually a 'dead zone' of scattered Low Cover with High Cover such as trees or rusting cars punctuating the area, followed by the back area of the map containing the spike that is your objective. Said spike is often beside a civilian structure, such as a barn, but can also just be standing in the open. I've yet to see it inside a building, and the way things are presented it would be pretty narratively bizarre for the spike to spawn in a building (Or more accurately to do so without having clearly smashed into the building from above), but I'm not willing to assume that's a rule the game actually holds itself to completely consistently.
The spike (Or 'Disruptor', though it's only referred to this way in the mid-mission objective description) itself is a destructible High Cover object whose only point of interest is that its destruction is essential. Specifically, your objective is to destroy the spike, then ideally retreat everyone to the Avenger and order it to lift off; anybody outside the Official Ramp Zone will be left behind, but this is worth keeping in mind if things are going really badly. Also, the spike always has a pod clearly assigned to guarding it -they won't move that I've ever seen, not before activation- though oddly they tend to be placed behind it, so it's often possible to destroy the spike without ever activating its guards. This is easiest to do by having a Concealed soldier spot the spike while Squadsighters snipe it, but even just a careful advance can manage to destroy the spike without activating its pod.
As you won't loot bodies and enemies will never drop timed loot in this mission, skipping such fighting is well worth considering. This is especially true because after a few turns, enemy reinforcements start dropping in unlimitedly, initially a flare every other turn but eventually every turn, which can make it dangerous to pick other fights. Among other points, this mission has the special quality that once its reinforcement pods are occurring every turn they are always led by a Heavy Mec (Unless you trigger this mission early enough they're not allowed to spawn yet), which are difficult to ambush with Overwatch, can soak a lot of punishment, are extremely dangerous to leave alive, and their preference for going into Overwatch on activation can put you in awkward situations where you can't move without risking being shot until somebody has hurt the Heavy Mec. This gets particularly important once you're retreating back to the Avenger, since the goal at that point is to just run.
Also, note that destroying the spike triggers this every-turn reinforcement behavior if you do it earlier than the turn they're supposed to start on. This isn't really a reason to not break it early, but it does mean you might want to delay finishing it a turn if that'll make it easier for people to turn around and run back to the Avenger. This is especially important to keep in mind if you're walking people up to the spike as a group to attack it from close range, instead of using a spotter while Squadsight attacks handle it; a spotter gets a full turn to run once the job is done, after all, and if they're a Reaper in Shadow they'll be very fast and able to ignore Overwatching Heavy Mecs as they run.
Note that if you want a Concealed spotter, you should deploy your Reaper or Phantom as part of the initial force, as the soldiers Bradford sends out will not start Concealed. (Admittedly Reapers can manually enter Concealment, but it's still better to actually use their on-deployment Concealment) Also note that Bradford usually sends out soldiers from highest rank to lowest (Though I've sometimes seen him elect to send out Squaddies when there was a Major still available; I'm not sure what causes this), with no regard to their Fatigue level; this is actually basically perfect in the base game, but can be awkward in War of the Chosen if you've got some high-level soldiers who need a rest. You may wish to pass out Mind Shields to any higher-level Tired soldiers before launching the mission, to avoid them freaking out in the mission if they get reinforced in.
Also note that destroying the spike interrupts Bradford sending out soldiers. This is another reason to hurry to destroy the spike in War of the Chosen, so as to reduce the Fatigue load. In the base game there's an argument for deliberately letting all your troops get deployed so they can all earn experience, instead... it's a bit awkward of the design, and I'm glad the Chosen Avenger assault doesn't repeat this reinforcements mechanic.
Also, if you have the Defense Matrix up, there will be 2 or 4 friendly Turrets, depending on if you have the Defense Matrix upgraded or not. In the base game this is cute but not terribly helpful, as only a very small portion of the combat is liable to occur close enough to the ramp zone that the Turrets have the opportunity to actually shoot at things. It's nice if an enemy decides to make a run for the ramp, and can be helpful in the final stages of retreating to the Avenger, but it's entirely possible to end up with the Turrets not firing a shot in the base game. You should be diligent about checking them first regardless; they can fire twice a turn every turn without worrying about ammo, so it doesn't really matter that they're prone to missing, you might as well go for the free chance at damage, given if it works out it'll free up your soldiers for other tasks.
In War of the Chosen, these Turrets are noticeably more useful, assuming you have the Defense Matrix manned by an Engineer, as they now pick up Squadsight when manned. This lets them take potshots at enemies throughout the map (Albeit with increasingly atrocious odds of actually hitting as you move farther out), and usually they can handle the duty of sniping the spike, where Squadsight penalties don't matter. (Sometimes you'll get a map where the Turrets don't have line of fire to the spike, unfortunately) Squadsight also lets you use them to smack Overwatching Heavy Mecs to let people run without risking being shot, which is very appreciated when you're at the step of running back for the ramp. It's even more important in War of the Chosen to remember to check the Turrets early in a turn and plan appropriately; among other points, don't forget that Holo Targeting and Aftershock Volt can bump up their hit rate against a target.
On a completely different note, this Avenger Defense has one of the best tunes in the game (Which actually also plays in the Tutorial version of Gatecrasher; the devs know it's a great tune), to the point that if, in War of the Chosen, I'm running one of the Tactical Legacy Pack music sets, I will switch away from them for the duration of the mission so I can hear this amazing music some more.
In general, while I think the gameplay experience of this Avenger Defense is very flawed and I appreciate War of the Chosen taking a stab at coming up with a more interesting variation, the presentation for this mission is actually really great. In addition to the music, I quite like the effect of the spike 'pulsing', where you can see and hear what it's doing from the opposite end of the map, and the cinematography of stuff like the opening bit with your troops coming off the ramp is surprisingly nice. As an experience, this is one of XCOM 2's best bits, enough so that I don't mind playing through it even though its gameplay end is a bit weak.
It's interesting to me that the Avenger Defense is where XCOM 2 has some of its strongest experiential design. I wonder what happened that led to this?
More narratively, this is a mission that is pretty straightforward in concept ("UFO shoots Avenger out of sky, enemies try to board the Avenger once it's on the ground") but if you scrutinize it is another case of XCOM 2 not quite having its story straight. For example, in the cinema that triggers when the UFO catches up to you, the UFO fires on the Avenger, cutting its power, and Shen starts out baffled as to why the power isn't working (She talks about there being no damage) and then arrives at the assumption that the UFO must've used an electromagnetic pulse, and in turn the implication is the spike is repeatedly outputting an EMP... but once you're in the mission, Shen's wording is uncertain, as if she has no theory at all for what's going on with the Avenger's power not working beyond 'the spike is doing it'.
(Mind, I prefer the latter scenario; video games having arrived at the idea of an EMP being a way to inflict a Stun status condition on electronics with no long-term harm is one of those video game memes that's not only highly unrealistic in general but that I suspect is actively mis-educating people, unlike a number of other video game memes that seem to be readily understood to be game mechanics drawing at most loose inspiration from reality; an EMP knocking out electronics is generally going to do so destructively. The initial attack and the spike's effects being Mysterious Alien Technology Doing Who Knows What is far preferable to having Shen insist an EMP is the explanation)
Bradford's conviction that this operation is an attempt to capture the Avenger is also weird, not only for reasons I've raised before (The lack of an obvious reason for them to care enough about a stolen Supply Barge to want it back intact) but also because the Ethereals were holding onto the Commander for years. The obvious guess is that they're trying to recapture the Commander; this especially stands out when playing War of the Chosen, where the Chosen have an explicit objective to recapture the Commander, making Bradford's conviction that the enemy forces are after the Avenger all the more conspicuously confusing.
The whole thing is functional enough, but it's another sign XCOM 2 was released before it was really ready.

Lost Towers

Something I didn't cover in those posts is that the leadup is a bit jarring, as the game kicks off this mission by, when the game decides to spawn the Rumor you need to scan to generate the Lost Towers mission, abruptly kicking you into a cinema with Shen getting the signal from the Lost Towers. It's a bit jarring, especially because there's nothing prepping you for the idea; War of the Chosen kicks a cinema off when it's time for a Chosen attack on the Avenger, for example, but you get warning at the beginning of the month that such is on its way.
It's not a big issue or anything, but it's nice that Alien Hunters didn't imitate this bit.

The Nest
I've already covered this in detail in the Neonate Viper post. But hey, here's the image used for it.