Loading Rooms

Can someone explain the process of having a loading room in a game, similar to the loading rooms you get on Steams-The Lab and others using blueprints.

I assume it is just another basic room ‘Map’ that is loaded player is moved to but how is the following level loaded while you are waiting in the room until the map loading has completed. Is this done in the loading room or set up in the map prior to the loading room?. I may be totally misunderstanding the concept behind how it works/loads, looked into Loading screens tutorial but this is aimed more at flat 2d loading screens rather than whole rooms.