Where do you find out how to program a specific microprocessor?
I assume you're talking about how to physically write to the program memory, and not how to write programs using an assembler or compiler for a given micro. 

Often this information is not in the datasheet or in the data that covers the instruction set, but in a separate document. 

In some cases, unfortunately, that information may be tightly controlled and you may have to sign an NDA (Non-Disclosure Agreement) and/or prove that you are a real company and have a legitimate need for the information. 

The assumption is that an ordinary moderate to high-volume user will merely buy a programmer (or get a supplier to pre-program the parts), and has no need to know the protocols involved. Many Asian suppliers really are not interested in providing support for a customer who only wants a few thousand pieces- their ratio of engineers to production workers won't allow for it.