Usenet is just another kind of Internet connection, much like E-mail. The difference is, instead of delivery to a private mailbox, Usenet messages are stored in public directories, where any user on any system may retrieve them. In practice, of course, there is newsreading software that sorts the messages according to group, subject, date and other headers, before displaying them to the curious user. In look and feel, the newsreader seems to be a proprietary BBS forum. But the newsreader is not Usenet.
Newsreaders vary from user to user and from site to site.
The messages are passed from site to site. Some are posted almost instantly. Others may take days to circle the planet. Some sites save some messages and junk others, but all messages manage to make their way around the planet. Just as anyone can send a message, anyone can create a newsgroup.