Mostly stuff around the house, so replacement parts (broken stuff, missing caps, etc.,) or useful crap like a pen holder that fits into the hole left in my ikea desk from one of their qi-chargers that turned out to be less convenient than I thought :-P Turns out having a 3d printer one tends to find use-cases all over, just like one does having a 2d printer. You just didn't consider those before you had one and now, poof, you can just make it when you have an idea.

I mostly do very technical designs, mathematical curves rather than organic ones, if at all. I'm a programmer so the concept of "writing" my models instead of 'drawing" them feels more natural to me, hence OpenSCAD instead of the usual CAD tools or even blender (it certainly helps that I did a lot of raytracing stuff with povray years ago). It ain't art, but figuring out the real-world strength of different geometries, how to design screw-holes that work even when sagging somewhat in one axis, creating an exact mathematical description of the thread for a nut and bolt that work despite the crude resolution of a FDM printer... all these tickle my brain and I enjoy them.

As to learning there are many decent tutorials on designing "production ready" parts (think small-scale manufacturing runs), e.g. "Slant 3D" on youtube. But ultimately my answer has always been "becoming fascinated, trying stuff out, and trying to find resources on specific problems I encounter" Not because it is fast or efficient, but because I tremendously enjoy the experience ;-)