YouTube Music is the best way to download Google Play Music because the YouTube library is better than the one available for Google Play Music. As of yesterday, Google has made it very clear by announcing that future devices and the YouTube Music system will arrive before the launch.
All new phones coming with Android 10 and Android 9 out of the box, transfer the Google Music app from YouTube to the Music app. However, Google Play Music subscribers and users can download the application from the Google Play store and enjoy their libraries.
But with this in mind, it is clear that Google intends to hit Google Play Music sometime sooner or later.