Without setting up a project to try this (looks complex 
) I’ll write down some of the ideas I have.
Using the half life video as example to explain my idea:
You can get creative with a post processing material and custom depth calculations. This is often used to make effects like seeing one thing through another (like outlines through walls).
Alternatively, you could do it without, possibly, or make a mix.
Imagine the room in that video as a box within a box. The outer box is what you initially see (the room, the furniture etc.), the inner box is a simpler model which transitions to white.
The transition to white is controlled by a material asset (shader), basically a simple white emissive material with an opacity mask controlled by a shape growing over time (until the entire surface is white). This can be done with math (there’s a time node, and you can make shapes and gradients with math).
The initial scene of the white door popping up could use the same technique. It’s just an additional plane inside the inner box. When the inner box is fully white, its outer box can no longer be seen because you are looking at the “walls” of the inner box.
There is no real level transition going on for a visual effect like this.
references:
Making the middle part of the material invisible - #4 by Roy_Wierer.Seda145