How to gain a Root for a device?

Now this question is not about how to root a phone using a tool or apps like framaroot, its about how to gain root access to a phone. How do actually developers come up with a root tool or a way to root a device?
Is there any tutorial on this forum anyone can link me so I can learn how to find a root or create a root tool for a device? What all things should I have to learn?