Leave it up to the player. If they want to build a shared factory, so be it. If they want to build on separate corners of the map, that's cool too.

Just place the players in the same game world.

With regards to trickiness of determinism and what not, just make it so there's an authoritative server running the simulation, and the other players/clients just get their source of truth from the server.

The server could be an actual dedicated server program that has no frontend, or it could be one of the clients in the game.