@am1t It's tough to wrap your head around.
The most helpful is probably Gargron's: https://blog.joinmastodon.org/2018/06/how-to-implement-a-basic-activitypub-server/
Shsmeless plugs: I put together a "presentation" in https://github.com/go-fed/presentations but it's pretty specific and high level. The site https://go-fed.org has a tutorial specific to go-fed, but at the bottom I put a bunch of links to different references that I found helpful when implementing. But all this is probably less helpful to you than Gargron's post.