While you cannot purchase music from YouTube Music, you can do so from another store and upload that music into YouTube Music. This way, you can have your entire music library in one place. Note that you can play uploaded songs in the background, without ads and offline with an ad-supported YouTube Music subscription. More on that here.

https://support.google.com/youtubemusic/thread/62843644/google-play-music-music-play-store-music-manager-are-going-away-%E2%80%93-everything-you-need-to-know?hl=en