How can I change a special app folder into an ordinary folder? Applications in MacOS are special app folders. I assume this is done by setting some special permissions. Its normally not possible to navigate into such special folder from within other applications: the open file dialog of the Finder does not allow it. 

I have one such folder with a very big file. The file is too big to copy outside its containing app folder. So ideally I want to 'transform' the app folder into a normal folder, so that I can access the contents from another (normal, GUI) application.
Packages (‘app folders’) are displayed as packages only in Finder. To open a package as a folder in Finder, right-click on it and choose Show Package Contents.

For all other purposes they are normal directories.

/Applications ❯ ls Safari.app
total 0
drwxr-xr-x@   3 root  wheel    96B  6 Dec 23:39 ./
drwxrwxr-x+ 190 root  admin   5.9K  3 Mar 15:26 ../
drwxr-xr-x   10 root  wheel   320B  7 Nov 07:23 Contents/