you can make it out of any kind of milk, it's just mostly made from sheep because that makes the nice hard high fat form that goes well in salad as diced cubes

in bulgaria and former yugoslavia white cheese, mostly from cows, is a very common ingredient in many national dishes, especially pastries and salads, bulgarians call it "sirene" and serb/croat is just "sir" - the bulgarians use the turkic derived "kashkaval" for yellow cheese where the serbs just call all forms "sir"

there is an epically tasty hard swiss cheese from Bosnia called Livanski Sir, made from usually a blend of cow and sheep cheese, and is a staple savory treat at christmas time

oh yeah, and there is another special kind of white cheese type product very popular in the balkans also called "kaimak" which is basically the stuff you start with to make any kind of cheese, very often eaten alongside cevapi, a blended mince sticks, usually charcoal grilled, that is super tasty, it's a little bit like cream cheese but better