I’m not sure, either, about the “first three digits are the digits of the zip code”.
It is true that the FIRST digit sometimes is the Fed zone you are in (Maine, 0, California 9, like the fed appeals court districts.
I was born in Ca, but my SS doesn't start with 9.
Both of my children got their numbers at birth. The middle two numbers are the year of birth. They were born six years apart, in two different states.