New York certainly would be more competitive if NYC was only D+15.
Exactly. NYC being the nation's biggest city isn't what inherently makes NY so blue; one could argue FL and TX are just as urban, but metros in those states are generally much more conservative.
There must be some self-selection factors involved. One can say the self-selection factors in New York's case are among the most heavily pro-Dem of any big metro in the US.
Def. I think largely white liberal areas in places like NYC, Seattle, Portland, Bay Area, ect tend to be the most extreme political sorts in the Country; the culture of those areas is literally just "progressive".
Many heavily black communities are politically the most lopsided in the Country, but I think that's more racial sorting moreso than political sorting.