how to back up my iMac to icloud

If you are asking if there is a macOS equivalent to iOS's "iCloud Backup", there is no such equivalent. So the quick answer is, "you don't."

Conversely, iOS does not offer macOS's Time Machine.

To learn about iCloud Backup for iOS devices, read What is iCloud Backup? - Apple Support. To learn about iCloud and what you can do with it, read What is iCloud? - Apple Support.

For Time Machine, read Back up your files with Time Machine on Mac - Apple Support.

+1 John Galt is 100% correct!