As in: how the 3d reality is translated into a 2d image in our brains. The absolute basics that never change of how human vision works and how understanding that helps draw 3d-looking shapes better. Kinda like some parts of this video:
https://www.youtube.com/watch?v=tod2qZnKZEQ

anyone got any decent explanations or tutorials on that? I guess it would fall under "perspective" or "form" or smth, but I don't know of any good explanations.