Every screenshot REPRESENTS a unity scene and they're laid out in the same exact way the actual unity scenes are laid out in world space, so that we can be 100% certain that every screenshot is an accurate representation of the unity scene. That way, when we move screenshots around or 'enter' the screenshots, we can be certain that it's 1:1 the same stuff. The screenshots are being updated by hitting a hotkey and the screenshots can be activated / deactivated in every single unity scene, so you can always have them active and check how everything connects together while you're working on a scene.
I THINK that just happens in realtime - As you walk around the world, the game is trying to figure out where you could possibly go next and starts streaming in that scene before you can even get there, so often times in the game, there's 2 levels at once loaded at the same time.