In case you missed it, a couple of years ago Mastodon made an animated introduction explaining how it works.
✅ Mastodon's official promo video is available on PeerTube at https://tilvids.com/videos/watch/887e7b95-a6b1-4df1-83fd-d5c47f17ec4c and on YouTube at https://www.youtube.com/watch?v=IPSbNdBmWKE
The same video is also useful for explaining federated networks in general.
