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.