Boeing is the final assembler of major commercial aircraft, but a Boeing doesn’t go anywhere without GE or United Technologies powerplants, for instance. Boeing Commercial has about 8,000 suppliers in the U.S., and does something like $13 billion in business with them.
Commercial aerospace never embraced vertical integration the way the Detroit empire-builders attempted. That may be one reason why they remained cutting-edge.
Japan does indeed build military aircraft—google the J-2, for instance, and is a major Boeing partner on the 787. Japanese heavy industry does not launch a competing finished commercial aircraft product because they have been nearly completely incorporated into the Boeing supply chain.
China, on the other hand, is quite likely to launch a small (c. 100 seat) commercial aircraft for domestic consumption and international prestige purposes in the very near future.