If the Nazis where white supremacists then why would they ally with people they saw as less than them? If the axis had won the war would they have betrayed Japan in the future? It seems like a weird partnership.
u/Lubyak has an excellent response to this question here, regardjng the Japanese perspective of a military alliance with Germany, and u/delamarche addresses the question of reconciling Hitler’s racial theories with the Japanese alliance here.
If you have any specific follow-up questions, let me know, but this should be a good starting place and hopefully these answers can de-muddy the waters.