What type of cheese is used on Patsy's Old Fashioned Margherita Pizza
You can buy firm, sliced [or grated, of course] mozzarella in supermarkets for sandwiches or pizza topping.
[I've never tried it, I can't imagine it would be to my taste] but I see no reason it couldn't be some commercial application of 'firm' mozzarella, as opposed to trying to slice the good stuff, supplied in its own whey, which I use for everything from tricolore/caprese to pizza topping.
Wikipedia describes this as &quot;Low-moisture part-skim mozzarella&quot; which sounds… ermm… delicious ;)

Low-moisture part-skim mozzarella, widely used in the food-service industry, has a low galactose content, per some consumers' preference for cheese on pizza to have low or moderate browning. Some pizza cheeses derived from skim mozzarella variants were designed not to require aging or the use of starter. Others can be made through the direct acidification of milk.