Tons of buildings aren't residential. And there's still lots of single family homes as you get out into Queens, Staten Island, up further in the Bronx and further out in Brooklyn sometimes too.

I wonder how many people are living in buildings with 100+ people as I am (estimate a couple thousand in my building from back of napkin math I think).

New York City's fairly vast when you take into account Brooklyn, Queens, Staten Island and the Bronx. Manhattan's less than a tenth of the land area.

People forget a lot of NYC looks like this:

https://maps.app.goo.gl/ch4rPVXcMoxsBLDK6

The biggest buildings tend to have a residential population of approximately zero.

One million buildings…that's residential, commercial office or retail, and other (factories, storage, etc).

yes, and somewhere there will be at least one very dry definition of what NYC considers a "building", but then again it might be contextual and turn out to be a deap dive into the mind twisting depths of beuorcratic moat's and silos, only for the very brave and commited