Hey. The pixelated art/textures were created (or in case of the doors and ceiling, photos) by me using blender, pixel composer, aseprite and gimp. I have since moved onto Affinity from gimp.
The PS1 / Shaky texture feel is using a CC0 shader that I had to fix and port to a newer version of godot from godotshaders.com, I then edited some values until I liked the feel.
But the look is primarily just using larger texture bases, then resizing it to a lower resolution.