I am considering to choose iCloud Drive vs Google Drive to store old music and photos folders present in external drives. Is it possible to upload large folders to iCloud Drive that are not present in my Macbook SSD, but in an external drive? What is the best option for this kind of purpose? May I use both services, each one for a certain task?
You can not selectively sync content explicitly. If you allow it in iCloud settings, it will delete local copies of large files–but this cannot be directly controlled. You cannot sync iCloud drive to an external disk.
You need a local copy for iCloud, so you may want to use Dropbox or Google Drive for your external drive.