What's the Purpose of a Multiculturalism?

I struggle to understand how we can ever find a definition of racial groups and divisions thatm is coherent enough to make sense. I don't understand what it means to be dumped into, or not dumped, but lumped into some enormous group like monolithic whiteness. What lynx a white anglo saxon protestant with a sicilian or a spaniard? Wy? How do we measure intelligence? And we've never even lived in a world where we really have seen what paroty looks like.