I always upvote content from Julia Evans, she's doing amazing things on many different subjects. And this is no exception.

That said, specifically about Git internals, I found that the "ugit: DIY Git in Python" tutorial [1] by Nikita Leshenko was never surpassed in terms of quality and clarity as to understanding the inside of the .git repository.

[1] https://www.leshenko.net/p/ugit/