If an instance goes down, then pretty all much all data on that server will not be accessible. So the communities, posts and users on that instance will be gone.

Maybe lemmy can implement something similar to the matrix protocol.

Matrix is really a decentralised conversation store rather than a messaging protocol. When you send a message in Matrix, it is replicated over all the servers whose users are participating in a given conversation - similarly to how commits are replicated between Git repositories. There is no single point of control or failure in a Matrix conversation which spans multiple servers:

So a community will remain accessible even if the homeserver goes down

So downloading a video as a torrent will reduce the load on the peertueb instance?