iCloud & iCloud Photos · iCloud Drive basics
How do I sync a folder like Pictures or Music to iCloud Drive?
Short answer: iCloud Drive syncs only what's inside it, plus Desktop and Documents if enabled. To sync another folder, move it into iCloud Drive. Leave Photos and Music libraries where their apps expect them and use their own iCloud features.
iCloud Drive only syncs what's inside iCloud Drive itself, plus Desktop and Documents if you turn that option on. To sync another folder, move it into iCloud Drive. There's no supported way to sync a folder from elsewhere in place.
Think before moving very large or app-managed folders: the Photos and Music libraries should stay where their apps expect them and use their own iCloud features. And syncing isn't a backup — see is iCloud Drive a backup.