You can now download all or part of the city of New York in 3D in a variety of file formats, from Geopipe completely free. They also have a Unity SDK although I could not make it work. Thankfully for Unity developers they have made a pair of city packs available on the Asset Store. Once you have selected the portion of New York you are interested in using an interactive map, you can then download it in a variety of popular 3D file formats including FBX, GLB, OBJ and more. This should make the results usable in pretty much every single modern game engine. The assets themselves are usable under the Creative Commons Attribution 4.0 International (CC BY 4.0) license.
Asset Packs on Unity Asset Store
You can see the free virtual 3D New York City from Geopipe in action in Unity and Blender in the video below.