Attentions oversimplified:

The usenet is a network of servers in which each has file snippets that have a ID. All the servers sync their file snippet database so that all servers have all the snippets (some prune older snippets).

You then make a subscription at one of of those server provider and are then allowed to download those snippets.

Your downloader gets a map with the info which snippets to get and how to add them to get a file

Those maps (NZBfiles) you get from indexers (preferably private ones), similar as it is with torrents.

The usenet was created and used to fight censorship in 1980 only intended for text at first, but later it was discovered, that one can use this protocol for file sharing as well. It is hard to take down because of it’s decentralized design.

It is like the the prehistoric blockchain, lol