Just a thought.....the 'files' you are opening in file manager which automatically mount and then are shown on the desktop, are from the "device" area of trhe file manager.....Yes ? (see screenshot below)
View attachment 19852
The inclusions in that red boxed area are not ordinary folders.....they are 'mountable' devices...hdd, ssd's, external drives, etc
It is an expected action for them to show on your desktop as mounted after clicking on one......there is a "assumption" on the part of the OS that you wish to work further with that drive etc
You will not get an automatic mount etc anywhere by clicking on any of the actual folders above that red boxed area.....eg if you were to click on Downloads, it will simply open the downloads folder....it will not put an icon on the desktop.