I'm not in the game industry at all, but, do you or anyone know how this goes? What goes into a game design? How much more detail and story and worldbuilding is there behind the scenes?

I mean if it's anything like software development, the answer will for the most part be "well there's actually nothing here". A lot of sequels feel bolted on in that regard, where they just took elements from the original game, world and story and build on top of that.