iCloud Drive and iCloud Photos are two different things.
To turn on iCloud Photos, see information in Set up and use iCloud Photos - Apple Support
If you aren't sure whether your photos are synced with iCloud, log into www.icloud.com and go to the Photos there. Any photos you see there are iCloud Photos.
Also see:
Apple offers iCloud Backup for iPhones and iPads but not for Mac computers.
It is possible to sync data from a Mac to iCloud if you turn on those options (e.g., iCloud Photos, iCloud Drive including the Desktop & Documents option). Syncing has some of the benefits of a backup (and the benefit of being offsite) but is not the same as a backup. Major differences include the fact that not everything on your Mac can be synced and the fact that if you delete something, accidentally or otherwise, there is only a limited time period (30 days in most cases) during which it can be recovered.
Also see the Apple article: How to back up your Mac - Official Apple Support
And more information on syncing in:
Set up and use iCloud Photos - Apple Support
and
Access and manage your files in iCloud Drive - Apple Support
and
Add your Desktop and Documents files to iCloud Drive - Apple Support