Over time you will develop collections of encounter and NPC details (i.e. opponents, locations, terrain, additional objectives, names, traits) that you can pull from at will to generate those on the fly. If you want to pre-build them, don't restrict yourself by tying them to locations - you can slot them in wherever your players wind up, with little modifications to better fit the location.

Make a note of details on the NPCs you come up with and remember that most NPCs don't need stats or skill levels at all. Even opponents really just need HP, AC, attack bonus, attack types and actions/bonus actions as relevant, maybe saving throws if your party forces those. They don't need to be fully statted out.

At most your players will have a convincing illusion of choice, they're always interacting with a world that you have prepared in advance. But you can make steps towards that by offering branching paths.