If you're looking for a very simple ActivityPub implementation that lets you post, follow, and be followed, I can point you at snac: https://codeberg.org/grunfink/snac2. It's 100% C, and not a lot of code at that. It should be easy to follow and debug, and you can double-check your implementation against it pretty easily with some choice breakpoints. And you can stand it up as an individual instance to have it talk to your code if you want to test interoperatability.

(I would not really recommend it for general use, unfortunately, since it's a pile of C that's not really all that secure. But as a publicity stunt we run @[email protected] inside of iSH itself, and snac turned out to be excellent for this because iSH is slow and doesn't implement all of Linux, so picking something simple and lightweight was a must.)