> Everyone is so mixed-up that "stick to your own kind" becomes nearly impossible.
This is about 180 degrees from all of my experience with NYC and New Yorkers. I find New Yorkers to be extremely tribal, and many of them are also extremely efficient at identifying folks from other tribes who may kind of look like them (at least from an outsider point of view).
To be fair, much of this tribalism may not be based on things like race or religious/church affiliation -- as you mentioned, that would make living and working in NYC extremely difficult.
That said, things like career field, social status, and how you spend your free time seem to matter quite a bit.
As a simple example, take your random high SES resident of the Upper East Side and have them go into a grocery store in a low SES neighborhood (race doesn't matter) and strike up a casual conversation with the cashier. It will be extremely awkward, and everyone will be looking at them. Flip the script and have someone from that same low SES neighborhood go to a grocery store in the nicer parts of the Upper East Side and do the same -- this will also most likely be awkward with everyone looking at them.
The example above uses two fairly extreme examples, but this happens along much finer lines as well. Where did people go to undergrad/grad? Where do people shop? Where do they go out? What neigborhood/building do they live in? What do they do in their spare time? All of these things are hyper-efficient shibboleths in NYC that largely do not matter in a random small town in the Midwest -- that is, you probably didn't go to college (one of the two state Us if you did), you shop at the only store in town or the nearest store if there are more than one, etc.