Backing up itunes from iphone 6s to iphone 1pro.

An iPhone can never serve as a music repository. You cannot transfer music from an iPhone to anywhere.

You have to figure out where your music is housed. You mentioned “iCloud”. Your music would be there if (and only if) you currently subscribe to Apple Music, iTunes Match, or both. If you do, then you may move music from iCloud to your iPhone 12 Pro.

I don’t believe so. This explains how to subscribe.

Subscribe to iTunes Match - Apple Support

I should add if your music was purchased from the iTunes Store, you may simply redownload it to your new iPhone from the Store.

Redownload apps, music, movies, TV shows, and books - Apple Support