You’re an inventor : Great PMs are usually creative types, who can generate and churn through many ideas all at once, to arrive at solutions to difficult and multi-faceted problems.
The Build It stage ends when management and the squad jointly believe that this product fulfills the basic narrative and is good enough to start releasing to real users.
Another one of the biggest benefits of a written PR/FAQ is that it enables the team to truly understand the specific constraints and problems that would prevent a new product idea from being viable and aligning on them.