I understood that cheese is made by throwing in Stomach hide into milk and letting it congeal.
How was kosher cheese made (where one's not allowed to mix milk and meat)? Now we use chemicals or microbes, but how was it done naturally?
Very good question with a simple answer. It used to be made from rennet obtained from the cow's 4th stomach. I recall that almost all Miller's and Migdal cheeses were made this way.
As this method has become costly and, perhaps, the market has been trying to cater to vegetarians, this method is no longer being used for kosher cheese, AFAIK.
I did see a Google reference to a Feb. 1991 article in Kashrus Magazine addressing this question. However, I could not locate this old article on the kashrus.com web site.