How do I manage map transitions?

I can’t seem to figure out how to switch between levels or maps in a nice seamless fashion. I’m probably overlooking something obvious.

The effect I’m going for is similar to the lift in Portal 2. Enter a windowless room and switch maps in the background while the player wanders about the room.

What I’m really after is something like the Star Trek holodeck or the TARDIS from Doctor Who, (where the transition room is its own level) but the lift is probably a good enough starting point.

It’s called level streaming.
Download Content Examples pack and open Level_Streaming.umap.

There are also several tutorials on YouTube:

So after going through that material, it looks like I need to set the transition area as the persistent level and can manually trigger the replacement of the sublevel based on my destination. Do I understand correctly?