A usenet client such as SABnzbd. This is equivalent to a torrent client like qbittorrent.
An NZB indexer such as NZBGeek, again equivalent to torrent indexers, but for nzb files.
And finally a usenet provider such as FrugalUsenet. This is where you're actually downloading articles from. (there are other providers listed in the photo in my other comment here)
Articles are individual posts on usenet servers. NZB files contain lists of articles that together result in the desired files. There are also additional articles included so if some are lost (taken down due to dmca/ntd) they can be rebuilt from the remaining data. Your nzb client handles the process of reading nzb files, trying to download the articles from each of your configured usenet providers, then decompressing, rebuilding lost data, and finally stitching it all together into the files you wanted.