@dpwiz@qoto.org yeah, I found that one, but it's more of a rabbit hole of details rather than a succinct introduction. https://socialhub.activitypub.rocks/pub/guide-for-new-activitypub-implementers is helpful, but somewhat disorienting.

That said, https://blog.joinmastodon.org/2018/06/how-to-implement-a-basic-activitypub-server/ and https://blog.joinmastodon.org/2018/07/how-to-make-friends-and-verify-requests/ are pretty close to what I was looking for. It conveys protocol's look and feel, so that I know what to look for next.