Most of the art is done in Koikatsu, with the faces done in Kisekae. The character sprites are generated from a program Oppi uses and then edited to fit the next update(s).
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Kisekae you can find online by searching and is a small program, and works straight out of the box. I'm unsure if you can add stuff to it but it's worth having a read for it. Koikatsu is a beast of a program to put it lightly; you're talking at least 200GB. To get it, like Kisekae, there's a website you can find with "Repack" in its name.
Specifically the version used by Oppi is Koikatsu Sunshine EX; it's not supported by the developers anymore (Illusion) as they went under and rebranded as Illgames.