The public network of NNTP servers. These servers replicate content, so public discussions are "hosted" in any particular server.
Usenet was originally designed based on the UUCP network, with most article transfers taking place over direct point-to-point telephone links between news servers, which were powerful time-sharing systems. Readers and posters logged into these computers reading the articles directly from the local disk. https://en.wikipedia.org/wiki/Network_News_Transfer_Protocol