Earlier quoted context omitted.
An important part about usegroups is that it is from an era of dial-in connections. One can dial into your ISP, sync News (fetch new messages in groups one is subscribed to and send messages) and then go offline and read/answer the messages without paying per minute and without blocking ports for other dial-in customers. This also worked between different news servers in distinct networks with unreliable connections.…
I see. It seems somewhat like mailing lists to me. I go online, my client sends and receives messages depending on which groups I'm subscribed to, and messages bounce around in a distributed fashion, somewhat like emails bouncing between SMTP servers. I still don't quite get the user experience. What if you're subscribed to alt.binaries? Do you then sync all the binaries when you go online? That wouldn't make sense.…
Yes. Your news reader (nee aggregator) would use the NNTP protocol to tell the server "please give me a list of articles in groups X, Y, and Z". Your reader would know you read up to article 234 in a group previously, but now there were 266, so there were 32 new ones. You'd tell the reader UI to go into that group, which would cause it to download the new articles (headers) and display them to you. If you saw a particular thread you liked you'd select that and the body of the article(s) would be downloaded.
If you wanted to do offline reading there was a file format call SOUP which would allow you to download it to your local machine (remember, this was often in the age of dial-up). There is no article on SOUP, but a similar idea was QWK for BBSes:
* https://en.wikipedia.org/wiki/QWK_(file_format)
To distribute articles/posts globally an NNTP server would talk to other NNTP servers: technically peer-to-peer was/is possible, but it was often hierarchical. One could also have site/local-only groups and global groups.