Keep in mind that, before the Immigration Act of 1924 just about closed America's doors, virtually anyone from Europe who could get to the US, and who didn't have TB or trachoma, could settle in the US, and not have to become a citizen. So, in 1943, a substantial percentage of people in America were immigrants, especially in NYC, which was the gateway to the US.

Immigrants tended to congregate among their own kind in NYC neighborhoods. That's how Little Italy, Chinatown, Yorkville, etc., got their identities--in those neighborhoods, almost 100% of the residents were of the same ethnicity.

For example, the Brownsville neighborhood of Brooklyn was called "the Jerusalem of the West" because, at its peak ca. 1930, it contained 295,000 people, of whom 280,000 were Jews. It was also the most densely populated neighborhood (1.5 square miles) in North America. Almost similar densities and ethnic solidarities were recorded in Little Italy and Chinatown. Basically, an Irish neighborhood was 100% Irish, a Polish neighborhood 100% Polish, etc.

WWII and its aftermath made NYers more mobile--they made more money, moved to better neighborhoods or to the suburbs. Gentrification completed the transition. Today there are no Jews in Brownsville, and very, very little left of Little Italy. Same is true of other ethnic neighborhoods in NYC.