How to minimize all other panels except the one I want to keep open?
With the active application, at the bottom of the application menu itself, you should have a number of options you can use, namely: Hide Application Name; Hide Others; and, Show All.

On an English keyboard you can use the following shortcuts:


commandH to Hide Application Name
optioncommandH to Hide Others


I'm not sure if it will be the same for you.

Regardless, using the example in your question, the simplest workflow to achieve what you want would be:


Open Terminal (to make Terminal the active app)
In the menu bar go to Terminal > Hide Others (to hide everything else, including the Finder's open windows)
Drag folder over to Terminal


By doing the first two steps you're basically making the Desktop visible, which is where your folder is, while also keeping the Terminal app open.

Of course, you should also be able to do the first two via keyboard shortcuts of some kind.

NOTE: - Once you're done, you can make all the Finder windows visible again simply by returning to the Finder. This works because you didn't actually minimise the windows to begin with.