YouTube Music is a music streaming service built around official releases, music videos, live recordings, remixes, uploads, and playlists. In practical terms, it is sold as a free ad-supported version plus paid membership options, so the product is not just the catalogue itself but the way the service packages access, apps, features, and account rules around that catalogue.
The point most consumers need early is that YouTube Music is an access service, not a permanent ownership product in the old retail sense. You are paying for the right to use the service on supported devices under the current plan rules, not buying a library that sits outside the platform altogether.