Thank you for the answer, it gave me a lot to go through (tried to look for tutorials on level streaming and world composition).
Neither actually look like what I need.
So, let me just ask much more specific question: If you would want to create a game that’s set inside a multi-story building, where you (as the player) always see just one floor (and you can switch between them), but your pawns would be able to traverse from one floor to another by stairs, how would you build such a world (in terms of used Unreal Engine building blocks and their structure) and what would you pick as a representation for the levels?