How do I recreate the Desktop folder?

Somehow a friend of mine has made a hash of the Desktop, Downloads and Documents folders on his Mac (running Big Sur). He doesn't know how he did it, but he moved all three into a DropBox folder and subsequently deleted the DropBox folder. All three of these directories now show pathways involving the nonexistent DropBox directory. I have been able to copy the contents of those folders down from Dropbox but I don't know how to recreate the symbolic link to tell Big Sur the correct path for any of these folders. So, is there a simple way to 'fix' the bad pathways that are currently in place for these directories and to restore the links to updated folders I have recreated with their correct contents? I guess it involves setting up symbolic links (ln) but don't know to do it. Any suggestions would be appreciated.