I was a USENET addict- long ago I had a shell account on "world.std.com" (The Software Tool & Die)- they were the first dial-up ISP, located in Brookline, MA. Anyway at some point they stopped- by which I mean they stopped billing me and people moved on to the web. But for at least a decade they left the machine running, and I could use it no charge.

So anyway, on a shell ISP, you read and post news with the "trn" command- "trn" is a text-only newsreader client.

For discovery- well USENET was in the filesystem. So "comp.arch" is located in /usr/spool/news/comp/arch. So you could just "find" and "grep" to discover new groups. Also there was a newsgroups file that had the full list- I forgot the name of this.

For the web today? I know there are for-pay sites, don't know any free ones. Maybe others do. Google groups was kind of good enough if you only frequented a few groups.

You can use https://www.eternal-september.org/ with a newsreader client. See: https://www.maketecheasier.com/best-usenet-newsgroups/