how do I sync the downloads folder

No idea if you are still looking for a solution but here is what I did.

1. Open Automator app on your Mac.
2. when prompted create a new 'Folder Action'
3. On the right side of the app there should be a dropdown menu which says 'Folder Action receives files and folders added to' at the top, change this to point at the downloads folder on your Mac.
4. On the left pane of the window, find 'Move Finder Items' and drag this to the right pane of the app.
5. in the 'Move Finder Items' action, set the target folder to the designed folder in iCloud i.e. ~/Library/Mobile Documents/com~Apple~CloudDocs/Downloads.
6. Command + S to save or file save

from this point forward everything added to the downloads folder on your Mac through say AirDrop or saved from Safari etc will now be automatically moved from the normal downloads folder to iCloud downloads folder.

really hope this helps.

I'm with WheelieNick ... if I have any file I feel might be useful on one of the other Macs, ity gets dropped into a folder in iCloud Drive.

I don't want Mac files and apps synchronized with iPads and iPhones.

Try and drag your downloads folder to your iCloud Drive folder