It uses P2P when multiple users is watching the same video. A PeerTube server can also mirror another PeerTube server’s videos and function as a peer.

You can see it this screenshot, that I’ve downloaded most of the video data from other peers.

PeerTube is build on ActivityPub, just like Lemmy. Right now federation is broken between Lemmy and PeerTube. When it’s fixed, you’ll be able to subscribe to PeerTube channels from here and comment as well.