Yes, sorry, I meant “Open Level”.
You would use Open Level to load the primary piece (the Persistent Level) of your “not physically connected world” and then load in pieces as necessary with level streaming. Think of it like this:
You OpenLevel to go to “Paris”.
The streets and buildings are part of the persistent level.
You go to walk into a house. Before touching the door, the inside of the house does not exist. You touch the door handle, then stream in the “house 1” level, populating the inside of the house. You look around, then upon leaving, when the door shuts behind you, “house 1” is then unloaded to free up memory, so you can go into every house and have them detailed without having to load the entire city of Paris before arriving.
As far as larger worlds being partitioned for performance, you’re most likely going to just have to be creative with occlusion/frustum/distance culling and not overload the system with rendering absolutely everything. Hills and buildings in open world games are functioning as performance boosters! 