https://www.cnet.com/news/samsungs-foldable-phone-screen-specs-7-3-inch-and-4-5-inch/

Samsung wouldn't reveal any further details about the handset's hardware, but did concede when I asked if the screens could be made of different materials. My guess is that we'll see Gorilla Glass cover glass on the 4.5-inch screen and a type of plastic to cover the interior screen. 

Apps will be able to work three different ways on the foldable Samsung phone's screen.