Who and how develops circuit diagrams of processors (Intel/AMD) with billions of elements?

Question

Electronics

The octa-core Core i7-5960X has a 356 mm² die with 2.6 billion transistors.

Answer 1

They are developed by companies that in the distant past created much simpler processes, then gradually improved over 30 years.
I personally doubt that there is at least one person in Intel or AMD who understands the specification of the ENTIRE processor circuit.
Also, like any complex project, for example, there is no person who understands the entire source code of the office or photoshop - there are people who have worked on some part for many years.