People are self segregating. In America some is by race and some of what we think is by race is really about cultures and subcultures or a combination of race and cultures.
In 1900 American cities had their German town, Greek town, China town districts. You had Irish, Italian, Polish, Black neighborhoods. In a city near me the Serbs and Croats lived in different ends of town. They all worked together in the mills and got on just fine, but they didn't live next to each other. The Poles were the buffer between them.
Cuban Americans, Mexican Americans, other Hispanic Americans don't live together unless there are so few of them that they have to. Because it has to be scary whether you were a German in 1850 or a Cuban in 1959 coming to a huge place with different culture and language. You tend to seek out your own people.
The culture stuff all breaks down in a few generations. Race takes longer but we see that finally breaking down too.