Well, Google is full of shit then. Because that isn't my program.
I think you are confusing supplemental work and assistance work. Supplemental work is done AFTER the main lift, but before the assistance. This includes FSL, BBB, BBS, Widowmakers, etc.
Assistance work is programmed via total reps done per the three categories: push, pull and single leg/core. There is a "base" assistance programming and this is modified based on what you do for your main lift and supplemental. For example, less total reps is done during BBB then when you are doing FSL.
It's a very basic example of the "push and pull" of training. You push something in, you pull something out.
Read this forum and the many answers on it. I can assure you that it has much more information on the actual program and what works that a random Google search. Go to the source, not the pretenders. There is a reason why AC/DC sells out stadiums and cover bands play for beer money.