As days go by and learning about this letterbox company in Hungary with a license to manufacture, the building part seems more clear.
I guess if BAC had a license to manufacture GA pagers they would get everything needed to make the pagers(BOM, Gerbers, firmware binaries,source code??), maybe even whole PCBs if the agreement was to just assemble them. They could modify the firmware to trigger on a specific broadcast message.
But then how do you make the Hezbollah source from the pagers you have ?
Would they wait for an order and swap the original pagers for the modified ones? or approach hezbollah somehow to get the order in the first place?
And the pagers blew up around the country, so they must have found a way to send the trigger message using Hezbollah's transmitters infrastructure. Or how could they transmit the trigger message over such a large area?
Anyway, really like a movie, amazing work of intelligence and engineering