Providers are companies and servers where the data is actually stored, as opposed to peers in torrenting. Since they are for-profit, you need to purchase a subscription to have access to these servers. r/usenet has some nice deals on those[2]. Frugal is usually the most recommended for beginners since it’s pretty cheap and has most files you’ll want.
Indexers are similar to what rarbg was. They make it easy to search for files stored in the providers. They usually require a subscription as well, and some require invites, but are really cheap and easy to get [3]. nzbgeek doesn’t require an invite and is pretty complete. nzbplanet and drunkenslug have more content, but require invites. You can get invites for those on r/UsenetInvites
Finally, you need something to download the files. There are many options available for that, but I find that the best one is sabnzb [4]. It is pretty complete and has a lot of moving parts, so I recommend following the trash-guides article on it to get started[5]
[1]: https://www.reddit.com/r/Usenet/wiki/faq/
[2]: https://www.reddit.com/r/usenet/wiki/providerdeals/
[3]: https://www.reddit.com/r/Usenet/wiki/indexers/
[4]: https://sabnzbd.org/
[5]: https://trash-guides.info/Downloaders/SABnzbd/Basic-Setup/
