iCloud syncing

See the information in the "Turn off Desktop and Documents" and "What happens when you turn off Desktop and Documents" sections of Add your Desktop and Documents files to iCloud Drive - Apple Support

The above addresses turning off the Desktop and Documents syncing but still leaves iCloud Drive syncing on (i.e., you can still sync just those files that you manually put in the iCloud Drive folder on your Mac.

Note that there is a section titled "What happens when you turn off iCloud Drive or sign out of iCloud" if you want to completely turn that off.

Initially , iCloud Drive and desktops and documents should be always turned on Use iCloud Drive to store documents on your Mac and iOS devices - Apple Support , the second step will be download all documents Work with folders and files in iCloud Drive - Apple Support

Note : stop syncing will not be the second step , it had always to be last step .

 Open system preferences > iCloud and uncheck the box of iCloud Drive a pop up window appears , showing to keep a copy of documents , and remove from the Mac .

( a ) if you had clicked on keep a copy , they will be archived in home folder that can be viewed by clicking on finder in the dock , take cursor on top menu bar > Go > Home > your user name > iCloud Drive ( Archive ) .

I've never tried that so I won't make a recommendation.