Usenet (or nntp) relies on physical servers at set locations around the world, these normally reside in large datacenters. these physical servers are owned by or maintained by the "backbones" so for example, Giganews owns and maintains its own servers, and sells access to them as either giganews, or supernews.com. They also sell access to 3rd party resellers who brand the access as their own and sell it onto you giganews resellers are RhinoNewsgroups, Usenet.net, Powerusenet - these may use their own cache for recent content, but once past that they all share the same content.

When a "linux distro" is uploaded to Usenet, it is uploaded to one of these physical servers, either directly through a provider like giganews, or through a reseller, this doesn't matter as these physical servers are all connected via certain propagation agreements with all of the other backbones and caches be it xsnews, searchtech limited, this process is called propagation, and it can take up to 40 minutes and even longer for these physical servers to become synced.

Here's a little diagram:

This is as you can see a cyclical process - the posters chosen provider gets the article, and then other servers go "oh what do you have that we dont? lets grab that" this process occurs until the servers are >=99% complete, when a poster uploads they normally upload their release with par files to account for 10% of the overall file - so if 5% of the release is damaged due to poor transfer or such - your download client can grab these par files and fix the missing sections - kinda cool!