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/
I just tried it with claws-mail. Here is a quick start for Ubuntu (this feels like 1999):
apt-get install claws-mail
claws-mail
it forces you to create a mailbox, don't worry about this (you can delete it later). Once you have the main screen:
Configuration->Create Account
Name of account: USENET
Protocol NEWS (NNTP)
News server: news.eternal-september.org
Check This server requires authentication
Fill in USER ID and Password you got from email after signing up with Eternal September and close.
Now right click on mailbox USENET, select subscribe to newsgroup.