Your iCloud backups – your photos, messages, health data, notes, etc… aren’t fully private by default because it’s not end-to-end encrypted. Apple employees, law enforcements, hackers that breach Apple can access them.
Turn on end-to-end encryption for your iCloud by enabling iCloud Advanced Data Protection in your settings
More on Apple’s Advanced Data Protection for iCloud: https://support.apple.com/en-us/108756