Whiteness is a pseudoscientific concept based on blood purity. Whiteness is defined not by what it is but by what it isn’t.
Like with the nazi example. Nazis don’t consider Ashkenazi Jews to be white. They have “impure blood” and hence can’t be counted as white.
Americans believe similar things about black people
https://en.m.wikipedia.org/wiki/One-drop_rule
It doesn’t make sense and it’s not supposed to. It’s all made up 15th century nonsense.