I know you shouldn’t need to understand how the #fediverse works to use Mastodon, but I’d love to learn some of the technical details. Is there a Fedi 101 somewhere that describes the basics of ActivityPub, how federations works between different kinds of services (like Pixelfed or Funkwhale), and what the possibilities and limitations are?

The intro of the specs is a great start: https://www.w3.org/TR/activitypub/
Looking for anything similar about the bigger picture of the Fediverse – including and beyond Mastodon.

Came across the insightful post "An ActivityPub Philosophy" by @cj@mastodon.technology. Provides a good conceptualistion of what ActivityPub is, what it isn't, what ActivityStream is, and how non-social-media apps relate to it. Website is MIA, but the Wayback Machine comes to the rescue: https://web.archive.org/web/20201124231343/https://cjslep.com/c/blog/an-activitypub-philosophy.

Next up: https://socialhub.activitypub.rocks/t/guide-for-new-activitypub-implementers/479