[terrortrain]

You don't need the extension.

I'm guessing a little bit, but this page almost certainly uses the web torrent project https://webtorrent.io/.

Basically, it's an implementation of the torrent protocol over webrtc.

The majority of torrent clients don't communicate with webrtc clients, but there are some that do (Like the official webtorrent client). So they act as a bridge between web page clients and regular torrent clients.