You can tell if files are the same across multiple systems by their hash, which is like a calculation of the 1's and 0's of their contents

Torrents describe a file by its hash, and contain a list of relays where users that have a copy of this file advertise it (like a nostr relay, but in torrent nomenclature they're called trackers)

Torrent is censorship resistant because anyone can have a copy of a file, and then advertise it on multiple trackers... since its basically impossible to shut down every tracker and copy of the file it's been unstoppable