How to back up music to iCloud

You can't.

You can re-download any music purchased from iTunes on the Xr at no charge, and you can re-sync any media that was transferred to the old phone from iTunes.

Music an other media are not part of the backup. It would be redundant to include it and unnecessarily consume cloud or disk space.

The iCloud Back up doesn't include Music.

If you are not using Apple Music, then use iTunes to back your iPhone 6 and then restore the backup on your new iPhone XR.