Personally I lean on real-world designs, honestly I haven’t taken advantage of some of the things that additive manufacturing enables over traditional machining so pretty much anything I’ve done could be done on a mill + lathe as well. Do lean on reference material though, dusted off my machinery’s handbook for some gearing references recently and I don’t doubt that I’ll get use from some of my other textbooks from uni, dated as they are.

I went through prusa’s documentation on designing for 3d printing when I was getting setup again as it had been some time, that covered things like overhangs, horizontally oriented holes etc. With all that said, I’ve always been an iterative design person, sometimes that’s also iterating on existing work, really appreciate the open source mindset in the community for that.