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)
Every website uses the http protocol, yet they are all completely different. Same idea with ActivityPub kinda. Also yes, you can actually reply to this exact post from Mastodon too which is pretty awesome.
But they share one thing in common. Thats the ActivityPup Protocoll, which allows them to somewhat communicate with each other.
All apps using the ActivityPup protocoll are part of the Fediverse and can somewhat communicate with each other. (Sometimes well sometimes not so well)