How does PeerTube work?

Doing my own research before asking here, I understand that PeerTube works similar but not the same as torrent protocol works and it seems to me that it uses the WebTorrent protocol, that means that I "share" fragments of the video I watch, and that the server although not behaving as a peer, provides the information to other servers.

Sincerely, even doing this research, I had many questions...

• What happens if a video has no peers? Is it simply unplayable?

• If a video has few peers, will only a part of it be playable?

• To share fragments of a video I need to be consciously in the video? I mean, in a torrent program I open it in the background and forget about it, but in the case of PeerTube there is no desktop program as such to share in the background the videos I have already watched.

• If I as a user share a video, and after a while I turn off my PC and I was the only one who saw it, no one else will be able to play the video until I turn on the PC?

• This will sound stupid, but why do servers have a storage limit per user if the server as such does not share fragments of it?

Thanks and apologies for the stupid questions.