can I delete files on my desk top to make more room on my hard drive once it on the cloud?

Files are not kept on desktop screen , rather in finder > iCloud section > Documents when iCloud drive and desktop and documents box is turned on Add your Desktop and Documents files to iCloud Drive - Apple Support

Use iCloud Drive to store documents on your Mac and iOS devices - Apple Support

Deleting the files will reduce less space from iCloud Delete files in iCloud Drive on iCloud.com - Apple Support

but it won't reduce Macintosh hard drive space , see the thread to reduce its space https://discussions.apple.com/thread/251789307

Note : you can also drag and drop some massive size files Access and manage your files in iCloud Drive - Apple Support to other location Work with folders and files in iCloud Drive - Apple Support

And also can use optimise feature Optimize storage space on your Mac - Apple Support

Free up storage space on your Mac - Apple Support

Use the "Optimise Mac Storage" option and let iCloud remove files from your Mac, that you have not used in a long time to free storage on your Mac. That is the only way to free storage, but keep the files in iCloud, if your Desktop and Documents are syncing with iCloud automatically.

If you are syncing the files to iCloud automatically, if you delete the file from your Desktop, it will delete from iCloud.