Looking around, I see a fair bit of both (1) and (2) happening side by side.
The best example of (2) in my mind is the emerging test suite.
I’ve seen this tutorial resonate very strongly with anyone who runs into it, and I think it deserves more widespread attention:
A dev partner of mine was having a hard time grokking AP in the midst of developing a Matrix-based app with all the mental mapping that entails. But with Seb’s tutorial it ‘clicked’.
OAuth/OIDC has a similar resource:
For people getting started with ActivityPub interoperability, I see the formal spec as something to be aware of, but not the place to start engaging. You should start where the “action” is at, and right now that’s Mastodon in the domain of AP microblogging and Lemmy in the domain of AP forums.