Believe it or not I didn't even know the backrooms was actually a game until a day ago Lol.
The pixelation is just a custom image effect shader on the Camera. For the pixelation I just multiplied the uvs of the render texture by X, round them to integers, then divide by X to get a pixel effect where X is how many pixels in the given dimension. For the noise I just had 5 noise textures I looped through assigning to the shader every couple of frames. The shader then just mixes everything together with different weights I can change via scripts to increase the intensity (hopefully that all makes sense).
I could send you the code if you're interested, but it's not very readable Lol.