More likely just plain mozzarella. In most restaurants, you're likely getting a factory pre-made frozen and reheated product from a box - certainly that was the

More likely just plain mozzarella. In most restaurants, you're likely getting a factory pre-made frozen and reheated product from a box - certainly that was the case at the chain pizza place I worked at in highschool. "Stuffed crust" pizza was made with string cheese (at least at the time), though. I've seen blocks of mozzarella chopped up at a fair to fry, but no idea whether that's what everyone does. The "stringiness" of melted mozzarella (as on a pizza or in a fried stick) actually mostly depends on the freshness, or so I've been told.