First, open iTunes on your Windows PC, go to File > Add File to Library, or Add Folder to Library. Then, choose the song or music folder according to the option you selected.

Upload local music files If you've been into digital music long enough to have a collection of MP3s stored away somewhere, you can add them to your YouTube Music library—up to 100,000 of them.

As explained by developer Ben Dodson, iOS 9.3 will give developers the ability to add songs to a user's iCloud Music Library. For an app to do so requires permission from the user.