I've seen this name but still don't understand where's its root of trust. In other words: - How does the program making use of it make sure it's not been patch

I've seen this name but still don't understand where's its root of trust. In other words: - How does the program making use of it make sure it's not been patched so the code that's supposed to run in the enclave runs without one? - How does it know it's not being run on an emulated CPU or in a VM? And that those checks haven't been patched out, too? - How does it make sure the OS is cooperating? I assume there is some cooperation required from the kernel at least.