Hi Smelly's Mum
Good question. Rennet is the slang word used in cheesemaking for the historical coagulant which indeed was made from animals stomach. The key component being Chymosin, which has been isolated and re-created using non-animal based microbial processes. I've built a short info page on it here
. My understanding is that most manufactured cheese today is made using this manufactured product.
Many cheese manufacturers websites have info on the type of the coagulant / rennet they use.
I guess a good question is if this modern genetically engineered product could be considered kosher, I expect there are varying opinions within the Jewish religion.
If you are making cheese I also initially used a Vegetarian Rennet from Whole Foods Stores chain in USA/Canada, picture on that webpage.
I expect there are many more info sources on the web, good luck!