Scott Chacon's videos are priceless. (He's the co-author of the git book.) They explain exactly how git works internally in the simplest terms possible. Either or both of these:

Introduction to Git - talk by Scott Chacon https://www.youtube.com/watch?v=xbLVvrb2-fY

Introduction to Git with Scott Chacon of GitHub https://www.youtube.com/watch?v=ZDR433b0HJY

The latter is newer but a bit longer.

Don't be fooled by the video names, these are introductions to the internals not just the interface.

I'm not OP, but I found this to be useful.

http://eagain.net/articles/git-for-computer-scientists/

Not the OP, but I have found the following useful:

https://git-scm.com/book/en/v2/ scroll to Chapter 10 "Git Internals"

(Direct link: https://git-scm.com/book/en/v2/Git-Internals-Plumbing-and-Po... but it only shows you the first page among nine.)

For a good deep-dive, I found this one very helpful: https://codewords.recurse.com/issues/two/git-from-the-inside...

Discussion: https://news.ycombinator.com/item?id=12802949

The official git page has very good documentation regarding git internals. (The whole book is worth reading) https://git-scm.com/book/en/v2/Git-Internals-Plumbing-and-Po... gives you a good overview on how git handles things internally.

The Git Parable: http://tom.preston-werner.com/2009/05/19/the-git-parable.htm...

Git from the Bottom Up is very good. Previous discussion: https://news.ycombinator.com/item?id=10199391