I think there are some tutorials somewhere for the basics, but really you shouldn't have difficulty just feeling your way around it. Don't hesitate to ask for help. That's where I think most who would would get stuck and give up. I can't really write documentation or tutorials, that's really something users ought to do for themselves, among themselves. It's just not a good use of time for tool developers. I.e. it's one of things anyone can do really. And really anyone using SOM should be thinking of ways to contribute. As for more advanced stuff, I can tell you how to accomplish anything that's possible, or make something possible that's not, if you have an impressive project. Really for advanced use you have to be willing to talk to someone.
RE downloads, I know I already sent you this (swordofmoonlight.net/ex/diy/) (sorry itch.io can't mix text and URLs in one post!) but just in case any readers have the same question, that's what you need. This is the only real mod that I know of, being my own. If you find anything else you'd need to be careful about being sure it's compatible before doing something with you work, since you might lose it. Make backups often of anything you intend to keep. If you want to make original 3D art then I recommend this (github.com/mick-p1982/mm3d/releases/tag/win32-dec-2021) system I'm using with my project that the MM3D models are formatted for. And keep up-to-date with patches I publish because it's a work-in-progress.