In contrast, Japan represents a small number of peoples, and it's laughable to suggest that someone can become Japanese or German by a mere act of bureaucracy.
America is really an immigrants country, and its easy to integrate there compared to countries which have less of a globalized culture, such arguably Germany and maybe the UK.
Anyone who thinks that it takes ‘magic’ to turn someone who was born in Germany or Japan into a German or Japanese person is implicitly subscribing to some bogus theory of racially pure nations. (See e.g. your sibling comment, by someone who’s persistently been repeating far right talking points over the past few months.)
I agree, there will be difficulties for sure, but immigration is at its easiest for children.