How do I backup macbook pro to iCloud

You can't back up complete Mac data to iCloud as it contains files from system library and user library .

Use time machine to back up complete data Back up your Mac with Time Machine - Apple Support

iCloud only saves data from applications when turned as on Turn iCloud features on or off - Apple Support

Read What is iCloud? - Apple Support

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

And more information on syncing in:

Set up and use iCloud Photos - Apple Support

and

Access and manage files in your iCloud Drive - Apple Support

and

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