MacOS – How to show the desktop from full screen apps

I would suggest you set up a hot corner to show the desktop (this is what I do):

1. Open System Preferences
2. Click Mission Control
3. Click the "Hot Corners..." button at the bottom
4. Set the corner you want to use for "Desktop" (I use the top right)
5. Click OK

Now you can move a file from a Finder window to the desktop with just the mouse:

1. Click and drag the file you want to move to the hot corner you set up, which will show the desktop
2. Drop the file on the desktop

As far as I know, mission control is set up in such a way that the dashboard and the first desktop are immutable (i.e. cannot be changed). There might be a flag or something for it somewhere that you could disable that behaviour, but it would definitely require some serious digging since it's part of how mission control is written.