It’s an old federated message board system. Message boards are called “newsgroups “. It predates the web so it’s usually accessed via a special client app. To use it you’d need:
A Usenet client app, called a newsreader. See Wikipedia. Many are probably abandoned by now.
An account with a Usenet provider. A search engine will point you to several options. There used to be some free ones. If there still are, it would be a good way to try it out. But note that the free ones often don’t carry all of the newsgroups — they omit the binary groups, which are known to carry pirated software and, let’s say, diverse video content.
It's federated? Okay, that makes a lot more sense. I thought usenet providers were like isps, connecting to a single, central host or something. I didn't realize they were federated systems.
If I recall correctly, ISPs were often running their own Usenet servers. This meant that traffic didn’t leave their networks and thus they paid less network interchange fees.
These days maybe only niche ISPs in some parts of the world might be running Usenet servers. Majority of them are run by specific companies created specifically for the purpose.
Since it may not be clear just from the content of your comment: ISPs ran their own usenet servers for the same reason we have CDNs today - they'd only have to transfer the data inbound once (which, as you say, costs a hell of a lot less money in interchange fees when you have a lot of users accessing a lot of groups).
A Usenet client such as SabNZBD (like qbittorrent or similar downloading client, but for Usenet)
A Usenet provider such as Astraweb, Newshosting, UsenetServer, Easynews, etc (or a paid subscription giving you access to a number of servers, kind of like a private torrent site)
And a tracker or indexer such as NZBGeek, NZB Finder, omgwtfnzbs, DrunkenSlug, etc (similar to a library index that helps you find what you want in the sea of information)
You can set services like these up with programs that use these tools to pull what you want automatically, such as the Arrs (Sonarr, Radarr, Lidarr, etc) or you can manually search either the provider directly through their own search engine, or through an indexer’s refined search engine.