**Sorry this is so late! I wrote this days ago but then came back and saw that itch.io had saved my reply, but I hadn't actually pressed "Post reply"!
For most of the backgrounds, I found reference photos on Pexels.com and redrew them at bizarre angles. I heavily edited all of them in Medibang: blurred them out, added chromatic aberration, inverted colors, added overlays with saturated colors or noise for fuzzy effect, etc.