How can I change a special app folder into an ordinary folder?
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/