Wrote a summary of web linking (AKA link relations; think rel="xxx" in a <a/> or <link/> HTML element).
The piece of the puzzle that's still unclear to me is whether WebFinger has been superseded, or maybe just generally ignored. There doesn't seem to be much adoption (except by Mastodon) or follow-on activity AFAICT.
I have some questions in regards to how #Federation / #ActivityPub account discovery works with multiple hosted services and a single account e.g. user@domain.com with mastodon.domain.com, pixelfed.domain.com, someotherservice.domain.com.