Everything you see is stored on their server, but certainly isn't owned by them. Usenet messages are shared using a peer-to-peer protocol, as Wikipedia explains:
Usenet is a set of protocols for generating, storing and retrieving news "articles" (which resemble Internet mail messages) and for exchanging them among a readership which is potentially widely distributed. These protocols most commonly use a flooding algorithm which propagates copies throughout a network of participating servers. Whenever a message reaches a server, that server forwards the message to all its network neighbors that haven't yet seen the article. Only one copy of a message is stored per server, and each server makes it available on demand to the (typically local) readers able to access that server. The collection of Usenet servers has thus a certain peer-to-peer character in that they share resources by exchanging them, the granularity of exchange however is on a different scale than a modern peer-to-peer system and this characteristic excludes the actual users of the system who connect to the news servers with a typical client-server application, much like an email reader.
You can find the origin of messages using the
Path header, which shows the origin server and the servers it passed through before it made it to your ISP's server. RFC 1036 explains this mechanism in detail.
Some newsreaders allow you to view all headers, while others allow you to view message source. Either way will reveal this information. However, keep in mind that Usenet headers can be falsified, just like e-mail headers.