I just want to mention that Mastodon has set the gold standard for this.
We take a lot for granted, like the MastoAPI, Webfinger, and sharedInbox.
These are not defined in the ActivityPub spec, these were contributed by Mastodon/Gargron.
The sharedInbox extension enables instances these large, which is a good thing, most people don't want to install Mastodon to have to use it.
@dansup Shared inbox is actually defined by the activitypub spec.