How are vulnerabilities searched for in devices, such as the iPhone? How does this happen?

Question

iPhone

Now I'm learning to code, I'm learning Qt, I thought maybe I'll learn something during this time, but I didn't know anything new in this matter.
What interests me is this: searching for vulnerabilities, for example, in the iPhone, for jailbreak. How it happens, what to read, and in general, some kind of educational program on this issue :)
Many thanks.