Usenet is a decentralized network that works very much like the fediverse here. Anything uploaded to one server gets sent out to all the servers; that’s why you can log onto lemmy.world and see posts made on say lemm.ee. Likewise something posted to one usenet server gets sent out to all the others. And these servers are being operated by individual entities (again, much like lemmy). Whoever owns the server you’re downloading from is having to pay for everything it sends out. Which is why most of the free ones will let you access the text portions but not the file downloads.

You do need a special program to connect to usenet. It’s built into a lot of older email clients (thunderbird is a good option).

There’s no one single usenet company for the same reason there’s no one single lemmy instance. (I’m sure someone has explained it better somewhere else in this post).