I once had a boss who was an Orthodox Jew and had a financial interest in a Kosher pizza shop, so yes it's a thing.
Being Kosher means being certified by a rabbinical authority. In the US, the Orthodox Union is best known for that. Having said that, Kosher restaurants are always either Meat or Dairy. A pizza shop is pretty much going to be Dairy so you can forget about meat ingredients.
One way to get certified is to show that all your suppliers provide Kosher ingredients. In the US there is a kind of milk called Xalav Yisroel, which is considered a higher standard. The pizzas at that shop were made with Xalav Yisroel cheese.
To be certified, the shop also had to show that it didn't do business on the Jewish Sabbath or holidays.
Even in large cities, there are usually very few Kosher eateries. Orthodox Jews very seldom eat out. This is one reason they love to travel to Israel because most restaurants there have Kosher certification even if most clientele aren't observant. So in Israel they can feel "normal".