What knowledge do I need to program digital art software from scratch?

I'm interested in learning more about how digital art software (such as Adobe Illustrator/Photoshop, Clip Studio Paint, Krita, Procreate etc.) works, especially since I want to make my own basic software. I didn't get much information when I tried searching up how to program my own software, so can you recommend some resources I could use? Tutorials, classes, textbooks, videos, anything? Thanks in advance!