Good afternoon DJMc,
Welcome to the Apple Support Communities!
I understand you would like to know how to use iCloud Drive to store files and folders in iCloud. You cannot store your Photo library or Music library files in iCloud. I would suggest using a local external drive to archive this media.
Set up iCloud Drive
iCloud Drive FAQ
"What types of files can I store in iCloud Drive?
You can store any type of file in iCloud Drive, as long as it's 50GB or less in size and you don't exceed your iCloud storage limit. That means you can keep all of your work documents, school projects, presentations, and more up to date across all of your devices. Learn more about managing your iCloud storage.
You shouldn't store app folders, libraries, or .tmp files in iCloud Drive."
Move your Photos library to save space on your Mac
"You can store your library on an external storage device, such as a USB or Thunderbolt drive formatted as APFS or Mac OS Extended (Journaled).1 Find out how to check the format of your external storage device.
To prevent data loss, Apple doesn't recommend storing photo libraries on external storage devices like SD cards and USB flash drives, or drives that are shared on a network."
Back up and restore your iTunes Library
Take care.