Someone asked a question like this recently, but didn’t have as many details about the book, and none of the answers seemed right to me. In the 1970s I read a book that was probably at least 10 years old by that time, if not much older.
It told the story of a teenage boy who, along with one or two of his friends, was going through the annual ritual of leaving his "world" and traveling through those of others. He went through a trapdoor into a farming world, where people were putting food onto conveyor belts that were feeding the rest of the worlds that, nobody remembered but the boy eventually learned, made up a ship that was traveling from Earth to another planet. The boy passed into a third “world” where people just sat around on electronic wheelchairs, growing fat while they did nothing but watch entertainment programs on TVs, and then finally to a fourth world where the scientists knew it was a spaceship and were wondering why the first world was not getting ready for the imminent landing.