Can you back up your Mac to icloud

No, there is no way to backup a Mac to iCloud.

You can however sync photos using iCloud Photos, and documents by either turning on Desktop & Documents in iCloud Settings or just moving them into the iCloud Drive folder in Finder.

Link-> Set up and use iCloud Photos - Apple Support

Link-> Add your Desktop and Documents files to iCloud Drive - Apple Support

These options sync the content, so anything deleted from the Mac will also be deleted from iCloud.

I highly recommend you have frequent and regular backups of your computer since iCloud is not a true backup of any pictures you sync there. Apple provides the Time Machine backup software for free to back up your Apple computer.

Back up your Mac with Time Machine - Apple Support