I've found this online course by Paolo Perrotta to be the best introduction to Git's architecture, by far: https://www.pluralsight.com/courses/how-git-works

He takes you pretty much from first principles all the way up to how remote repositories are tracked. Its been far more useful to me than simply trying to learn the CLI.

I realize its behind a paywall, but I'd recommend signing up for the free trial just to watch this course (if you're curious about the inner workings of Git).