how to back up mac book air to cloud

Like taylormark1234 wrote, you cannot backup a Mac to iCloud the way you do with an iOS device but it is possible to sync/upload a good range of files to iCloud:

Add your Desktop and Documents files to iCloud Drive

https://support.apple.com/en-us/HT206985

If in addition to Desktop and Document files you use iCloud Photos Library, Apple Music, Books, Contact, Calendar and the other iCloud, most if not all your personal files will be saved on iCloud. However, it would NOT be a backup as we traditionally understand it. For example, your system files and applications would not be backed up.

The best and easiest way to backup a Mac is to use TimeMachine.

Back up your Mac

https://support.apple.com/mac-backup

You can not back up a MacBook Air to iCloud, you can only back it up to an external drive, or a network attached storage device.