A newsgroup article propagates from one server to another, starting from the server where it is first posted. This means that when you post an article, the final result is tens of thousands of copies, all over the world. Eventually every server that carries the newsgroup has a copy.
Most servers normally send articles to other servers more or less in the order of arrival.
News server administrators make arrangements among themselves about which newsgroups they exchange. There are two methods. Most servers do this based on the number of days an article has been on that server. It might be a few days, or it might be two weeks or more. So, even after an article expires from your own server, it is probably still visible on many other servers.