Originally over UUCP (Unix Unix Copy Protocol) and done via dial ups at night (when the rest of the batch transfers were done - email too with the old bang path). The two servers would exchange all the batched email and news posts that were routed to the other side.

RFC 977 ( https://www.w3.org/Protocols/rfc977/rfc977 ) has an example of how files are copied between the two systems (section 4.6) including fetching and receiving mail.

Note that not all posts outbound are necessarily of interest to the other server. An IHAVE message could come back with either a "I want it" type response or a "not interested"

> The IHAVE command informs the server that the client has an article whose id is <messageid>. If the server desires a copy of that article, it will return a response instructing the client to send the entire article. If the server does not want the article (if, for example, the server already has a copy of it), a response indicating that the article is not wanted will be returned.

That's how some of the moderation worked - your server would say "I don't want anything that came by way of X host" or "not interested in that newsgroup."

One of the amusing things to me (looking back at this), if you're familiar with HTTP response codes, you'll likely get most of the way through the NNTP ones.

   200 server ready - posting allowed
   400 service discontinued
   411 no such news group
   500 command not recognized