How do I get my Documents and Desktop items back on my iMac from iCloud?

Have you tried going to iCloud.com/iCloud Drive using a browser, selecting the files, and then use the cloud symbol with the arrow pointing down to download the files?

Also check System Preferences/iCloud (Apple ID for Catalina or later)/iCloud Drive/Options to see if Optimize Mac Storage is checked. If it is, try unchecking it to see if the files sync back. The sync rate is very, very slow.

You are welcome. Glad to help.