photo booth how to move pictures to a ''useful'' location ?

That little symbol at the lower right will allow you to add the selected image to Photos, and other sharing targets. Like iPhoto and Photos, there is a Photo Booth Library in your Pictures folder. Inside that Photo Booth Library, you can find all images taken in an internal Pictures folder.

Click on your Desktop, and then press shift+command+G to open the Go To Folder panel. Copy and paste the following path into that dialog, and then click Go.

~/Pictures/Photo Booth Library/Pictures

You will land amongst all of your Photo Booth Pictures, which you can then copy, or move to other Finder locations. The file extensions are hidden, so you can select all of these images, and then press command+i. On that Get Info panel, uncheck hide extension. The .jpg file format is normal for pictures as it was developed by the Joint Photographic Experts Group (aka jpeg).

From PhotoBooth.app If you Control click on an image—>Export

https://support.apple.com/guide/photo-booth/welcome/mac

It's bringing you inside the library packag. A package is a special kind of folder that looks like a file from the Finder. It's designed to discourage the user mucking about with the internal parts. The Photos and iPhoto libraries are/were the same.

However, how do I insure i do not duplicate or triplicate stuff in such a ''virtual'' folder ?

I am not sure what you mean by that question.

Is there a way to enter that directory and just copy these pics (assuming these are not in a useless format) and just paste in another folder (even on a removable disk) ?

No. Or at least, none that anyone's ever found.

I doff my cap to you sir... don't know how many years I've looked for that....

🍺!

You are welcome.