With current technology, folding displays can either fold inwards (i.e. mating the screen surfaces) or outwards (mating the back surfaces). Either operation effectively doubles the screen real estate.
Samsung Galaxy Z Fold3
Huawei Mate Xs
No manufacturer can yet make a screen that folds both inwards and outwards along the same hinge. But Samsung has figured out to make a screen that can fold both ways along separate hinges. This means that they can triple the screen size, as compared to the object in its folded-up form: