How do I back up my iMac to the cloud?

As mentioned, iCloud is a sync service and not a backup.

Get an external drive and use Time Machine, CarbonCopyCloner or SuperDuper to make a backup.

Back up your Mac with Time Machine - Apple Support

https://bombich.com/

https://www.shirt-pocket.com/SuperDuper/SuperDuperDescription.html

Then plug the external drive into it new iMac and Migrate from the backup during the initial setup.

Transfer to a new Mac with Migration Assistant - Apple Support

Note: if the external drive only has a USB-A cable, then you will also need a USB-C to USB-A adapter.

USB-C to USB Adapter - Apple

Back up your files with Time Machine on Mac - Apple Support

No equivalent iCloud backup service exists.

Also, a cloud backup can take *days*, while an attached external hard drive can be pretty snappy.