Yes you need to subscribe to a paid usenet provider to download anything from usenet. You would configure its server details in your usenet downloader software (SABnzbd / NZBGet).
The .nzb you get from your usenet indexer helps you locate binaries you want to download via usenet. You can manually load those into your usenet downloader software or use automation tools to automatically do it for you. (Sonarr, Radarr, etc.)