Apple is not ready to make the jump to foldable designs for the time being, but luxury modification company Caviar from Russia might be able to inspire the Californian giant with its impressive iPhone Z concept. The Z-shaped folding pattern allows for quite a few interesting uses, including a laptop mode, a selfie mode, and a screen mode.
https://www.notebookcheck.net/Impressive-iPhone-Z-foldable-concept-estimated-to-cost-US-3-260.418216.0.html
Both the Galaxy Fold and the Huawei Mate X has visible crease when the display is off.
Very likely the reason is when its "folded" the screen would otherwise stretch, and they added bit more material so it doesn't have to stretch. The crease you are seeing is the extra material.
While this concept looks impressive, its not going to work. Look at how smooth and flat is the curve! When its fully open, you'll see the crease again. They could try making a display that has properties similar to rubber so it can stretch, but not only constant stretching and unstretching will have durability concerns, making a display that can stretch or compress a bit without impacting display quality is difficult, if not improbable.