Ask HN: Detailed step-by-step Git checkout

I'm trying to figure out a weird issue I'm having with pygit2 and checking out a remote branch in a git repository. The code examples I've found so far don't seem to do what I want, so I am trying to get back to basics:

I'd like to follow a step-by-step of exactly what git does during the command "git checkout".

The articles I've found so far either deal with other commands, or they gloss over what seems to be important steps.

Does anyone know of a good, step-by-step, detailed guide to the internals of git? Specifically git checkout?

Ideally it would detail what action is taken on each relevant entry under refs/remotes/origin/ and then refs/heads/ because that's the bit that doesn't seem to get mentioned.

I hope someone can help!