At the very start of the game you follow the overseer out from the vault. Her mission, she records as given to her by Vault Tec, is to "secure" the nuclear silos of post-war Appalachia for her employer. Somewhat suspiciously, given Vault Tec's reputation to the player and the explicit instruction to do so regardless of the state of surviving government, but otherwise unexamined as she presents it at face value of securing the sites for the good of the area, with no mention of what Vault Tec wants to do with them. That allows following the overseer's journey to be set as a goal without any onus on the player to commit to a specific course of action (and the quest reflects this - it's "Follow the Overseer's journey to see how she did", not "Secure the silos for Vault Tec").
The main quest then, roughly and if coming from the most likely "hero" roleplay perspective, sees you introduced to the various factions of Appalachia in an order of escalating ideological compromise and stakes in the world's survival, and this reflects the main questline and the player's journey.
At first you come across the Responders, an independent group of literal first responders who tried to just look after everyone after the war. They're pretty much as close as you can get to the "good guys", but that is also seen to contribute to their collapse - their capital devastated by Raiders, forced into an uneasy alliance with an increasingly demanding Brotherhood of Steel, and facing an ever escalating threat from the Scorched.
The first compromise made is heading into what was Raider territory. Finding one remnant of the Raiders left operating, you are forced to work with them to achieve your goals, as the Responders had tried and failed previously. It's not much of a corruption though - the Raiders are all-but dead, the single survivor a robot who seems to not fully grasp the situation she is in, and so assisting her doesn't seem to cause any actual harm.
Next is the Free States, a loose coalition of survivalists and liberals who themselves were conflicted after the war between keeping to themselves or helping those who previously had berated but now needed them at Harper's Ferry. By this point the threat from the Scorched is made much more clear, and your quest to complete the Free State's work hangs heavier. It becomes clear this is a last chance for Appalachia. Like the Free States though you are doing what's right - when they chose to emerge from the safety of their bunkers to help those in need, it might have gone against their personal beliefs and best interests, but it was the right thing to do. Unfortunately, you reach Harper's Ferry to discover that despite their assistance, things went south anyway.
To reflect at this point on who you have discovered: The Responders, ideologically pure and committed to helping everyone, betrayed by those they tried to work with. The Raiders, only out for themselves and long dead because of their reliance on others solving the bigger problems. The Free States, who achieved the most with their Scorched Defence System only by reaching out and choosing to concern themselves with the outside world for the good of Appalachia - but still they all died.
Then you reach the Brotherhood of Steel down in the Cranberry Bog, the source of the Scorched and the most dangerous of Appalachia's regions. All of the player's exposure to the Brotherhood so far has been through a lens of suspicion or outright hostility, and past Fallout familiarity is enough to know of their uncompromising militaristic approach. In 76 you see more of their origin from the US armed forces, and once in their territory you get a more sympathetic view on their actions. The Brotherhood were caught in a losing war of containment with the Scorched, and that pressure they placed on the Responders that once seemed unfair now seems necessary for all of Appalachia's survival. You too are now regularly fighting the worst of the Scorched, the world openly hostile in a way the almost pleasant starting forest region never was. From the Brotherhood it becomes clear that stopping the Scorched plague is not just a priority for Appalachia, it's a vital task for the survival of the world as a whole. If it isn't contained here, it will spread across the continent and all survivors will be overcome just as all the factions of Appalachia were - including the military might of the Brotherhood of Steel.
Finally then you reach the same conclusion as the Brotherhood. Containment has failed - the only option remaining is to completely destroy the source of the Scorched. Conventional warfare has failed there too - the Brotherhood pretty much wiped themselves out in an all out assault. But you have another option. You know how to find the nuclear silos.
So you come to the Enclave. If the Responders were the good guys, it's pretty clear these are the bad. Something previous Fallout players already know, but made very clear here too. This is an insidious shadow government, open about using you for their own ends, explicitly inhuman and untrustworthy. The Enclave's AI laments how it was locked out from the region's nuclear arsenal, and from the military satellites above the planet. It's pretty clear giving them that access is a bad idea. But you do it anyway, because it's only working with the Enclave that you'll be able to stop the Scorched and save the world.
And... that's what you do, I think? I haven't finished the final quest, of actually launching a nuke at the source of the Scorched plague, but that's where you end up. Launching a nuclear missile is the final option left open, reached after all others have failed - and you're doing so now explicitly with the help of the Enclave. Whatever noble intentions you set out from Vault 76 with, the world has forced you down this path of corruption. While you are saving the world, it's explicitly coming at great cost.
There is no player choice, because none of the game has been about player choice. This is a prequel - we know the Scorched plague doesn't devastate the world. This is the story of how that happened. Given the end result of working with the Enclave, I'm pretty sure it isn't an optimistic hero's journey, but an actually quite interesting story about how a survivor of a post-nuclear world ends up thinking it a good idea to fire another nuke. It's not a decision reached lightly, in the context of the story.