If you are going to use NZB's you will need access to newsgroups and indexers. Usually, these are paid for or invite only services.
An NZB is an XML file containing information about the file such as its name and publish date and also links to the segments of files.
A file uploaded to a UseNet server is split into smaller segments, binary files. The .nzb contains links to each of the binary files. When you download an nzb, it will download each binary in turn then add them all back together when all segments have been downloaded.
Imagine sending a 4GB file via email. It simply wouldn’t work. Split the file into 400 10MB files, compress them by zipping them up, creating an index file for all the zip files and emailing them one at a time. Then unzipping all the files and un-splitting the file. That’s an simile of how nzbs work.
A newsgroup hosts links to the files (nzbs) you want to download, an indexer files and sorts them in a nice order also providing an API allowing you to automatically search for and download files.
For example, you want to download a TV series. You add the series to your download client (Sonarr), connect the client to the site via an API key. When the series has been aired, via the API, the client searches the index for the file. If found it's added to the downloaded queue and downloaded from the location in the nzb file.
Who you choose to download from or index with is your choice. I'm listing who I use, I'm not recommending anyone.
• Search online for "top newsgroups" - I only use one provider, Aastraweb but most like to have a primary and a backup • Search online for "best NZB" - I use SU, CAT and OZ. Most sites have open registration however some are invite only • keep an eye on forums and the sites main page for open days
Usually you can pay for different levels of access, I pay for whatever the top level of access is purely for laziness and ease of access. You could probably spend some time and work out what works best for your needs.
When choosing bear in mind the sites retention, API rules and longevity, some sites come and go, others stick around.