Holy forget. I've never thought about heads that way before. All this time I've been drawing them as weird sort of rotated upside-down egg type shapes or even just circles with features and never really thought about why. The whole "upper head/skull" versus "lower head/face/jaw" really puts it in perspective
Also this is genius
if that method cleared things up, check out
this video for a further explaination. Like you said, drawing heads is like, 1000x easier once you seperate it into a cranium part and a jaw part.
the video makes it seem all technical and precise (which might be favorable to some people), but honestly, once you start drawing it, it becomes a whole lot easier. that's cliche advice, but it's also the most true. eventually you start learning how to simplify it even further, so heads become no big deal.
(how i do it, in case anyone wanted to know *shrug*)
edit: also, make it a habit to draw heads in different ways, with different features and expressions. once you learn about all the bits, and how they can change, it makes drawing different looking heads not so hard and prevents 'samefacing'
