We use "which" as a determiner to ask a question about a specific group of people or things:
我们用限定词“which”问特定的一组人或物：

Which restaurant did you go to?

Which countries in South America have you visited?

When we are asking a general question we use "what" as a determiner:
用“what”问较为普遍的问题：

What films do you like?

What university did you go to?