WP MU - how are users arranged?

Question

CMS

WP and WP MU have their own characteristics, incl. and in users/users.
Self-study, modest tests left doubts about the correct understanding.
Can you please tell us how WP users differ (in their device, so to speak) from WP MU users?
PS- I know that in MU the user regenerates once in the network (that is, it is not necessary on each sub-site), I know that the user in the network may or may not have his own sub-site. I know that for some reason you can add a user to a specific site ... and everything that I know seems to be.
Is it possible to immediately register on a specific sub-site (and not just on the network in general)? How? What does joining / inviting a user to a particular site give? Can you get him out of there? etc....