One thing that continues to amaze me is why during WWII (after Germany declared war on the US following Pearl Harbor) the Germans and Japanese didn’t collaborate more closely on weapons development and commonality. The Japanese could have built variants of the lighter German Panzers and adapt them to a tropical environment instead of relying on their own inferior designs. They could also have benefited from German plane technology once the Zero became obsolete. Conversely the Japanese torpedoes were far superior to their German equivalents.
Looking at the Allies, the UK and US shared technology very early on (jet engine, radar, sonar, bomb-sights), which greatly benefited both sides. The Sherman Tank was another example when fitted with a heavier British gun (Firefly), it proved one of the few platforms capable of defeating a Tiger head-on (responsible for the death of Michael Wittmann).
One reason I believe for this lack of cooperation is that the German and Japanese relationship was one of convenience and may have ultimately been a temporary one. Both cultures had strong ideas of racial superiority, which would have eventually lead to conflict had they both been successful in winning there respective campaigns. The exchange of proprietary information was not therefore as open as that witnessed between the Allies.
yes, the Japanese refuse the Bf-109 offer by the germans, so maybe what you say is true. But to say on contrary, the Japanese Zero were indeed superior then the early war American design. for some time, IJN pilots enjoy unchallenged air superiority over the pacific. only after the appearance of Corsair and the Hellcat, the table was even out somehow. But you can't say that Japanese design board was idle. by the second half of the war, several new design indeed came out of the factory, like N1K-J fighter aircraft which could effectively challenged the Hellcat. of course, just like Me-262, the design came out too few and too late to have any lasting effect on the war.