backup older iMac on icloud

iCloud is a conduit to have files on iCloud and available to Other Devices you have as long that they are All signed into the same Apple ID and iCloud is enabled

iCloud was never designed and intended to a to park Data like to Archiving data and make space on the Internal Drive

Really you really follow previous advise and purchase an External Drive with at least Twice the Capacity of your Internal Drive

The use Time Machine Backup built into the Operating System

You can't back up a Mac to iCloud. (Never could, actually).

You can get an external hard disk and back up your iMac to that via Time Machine...this is probably the best method for Mac backups.