If you want to upload videos (you want to be a PeerTuber, he, he), just subscribe to one (any one is ok).
If you want to watch, vote and comment on videos, there’s no need to make an account. Give the channel a follow from a Mastodon account, and simply interact with the post on your feed.
Any upvote will be reflected in the PeerTube channel. And the same is true for the comments: comment in the Mastodon post for the video and you’ll see the comment in the video’s PeerTube page.
So, you can follow channels from different PeerTube instances without a problem from a single place (your Mastodon account).
I’ve never tried it, but I guess the same is true for a Lemmy account: subscribe, vote and comment will be reflected on the PeerTube channel.