It is Boeing’s aircraft. Absent a sophisticated act of intentional sabotage, Boeing is at least partially responsible. The physical design, manufacturing, assembly, qualification, maintenance plans, and maintenance training are all responsibilities of the prime contractor. If a damn door falls off in flight more than one of those definitely contributed. It isn’t only Boeing. The FAA is also reasonable for verifying and requiring Boeing do all the right things and they will share blame as well as likely one or more of the several groups that will have contributed to the failure.