I’m confused, MAC, x86, Intel, Microsoft.

When i studied “computer architecture and organization” course in Collage, I learned that MAC computers have different architecture than PC computers (x86). And every architecture has it’s own instructions set. So, programs running on MAC won’t work on PC. I have this question in my mind,Since Apple started to implement intel processors in thier computers. Are intel processors in MAC computers are the same to those in PC? Is the Windows written to run on MAC the same one written to run for PC? A friend of mine found a pirated copy of MAC leopard OS for PC computers, Is this possible? or is it just a linux version with MAC OS theme? would someone clarify this to me.