Both are federated right? Getting a bit confused - are they interchangeable? Can I view lemmy posts/communities in Mastodon? Similar to how I can view lemmy.ml content with a lemmy.world account?
Lemmy is more like Reddit, Mastodon is more like Twitter.
In other words: Lemmy has communities (subreddits) and hierarchical comments for each post. Mastodon doesn't have either of those things, but it has following users and following hashtags.
Despite being different, they have some interoperability because they use the same federation protocol.
But you need to host the videos on your instance and foot the bandwith bill, which will prevent it from taking off like Youtube. Too expensive for anyone outside of gigantic monopolistic companies.
It relies on peer-to-peer, not hosting.
BTW: there is also a Facebook equivalent (Friendica, GNU Social, Dispora and Hubzilla), a Instagram equivalent (Pixelfed) and a Instant Messenger (Matrix)
Worth noting that while Matrix is decentralized, it's not part of the Fediverse!
The Fediverse/Activitypub protocol cannot (currently) be end-to-end encrypted, so it's not so great for secure direct messaging. Matrix is completely separate software for a different use case, but it follows similar guiding principles of open source and decentralization. :)
(Oh, and for book lovers, check out BookWyrm; if you used to love Grooveshark back in the day, give Funkwhale a spin!)