they used https://creator.nightcafe.studio/
i know this because we were talking about it in discord, mesw is the one who brought it to my attention :D
fair warning, once you get into it, it will suck up several days of your life while you play with it. check out https://www.artbreeder.com/ too.
mesw knows alot more about the technicalities of this stuff than i do. i just know it is hella fun to play with, and can be really useful for making background images.