How To Code Interior/Exterior Cell/Worldspaces Like Fallout/Skyrim

What I mean is, using seperate 'shell' models for the exteriors of buildings, with doors as seperate objects that go in the spaces required outside to let players in, w$$anonymous$$ch would then take them through a very short loading screen into the interior, w$$anonymous$$ch is a different worldspace or 'cell' or whatever you'd like to call it, spawning them in front of the door.

I'm not really sure how to word it, however I t$$anonymous$$nk anyone who's played either games will know what I mean, or any from the Elder Scrolls series or Fallout series (any of the 3D ones at least). I don't know the name for the feature, so sorry if it's a bit confusing.

Basically I just want them to press E or F or whatever to activate the door, go through a brief loading screen, and into a new 'cell' where the interior of the building is located as a separate model from the exterior shell, w$$anonymous$$ch is located in the main worldspace.