"The girl lives in New York."
Translation:Puella Novi Eboraci habitat.
Novi Eboraci is the locative form of Novum Eboracum. The locative tell you the location where something located. That's why it ends up being translated as in New York.
Note that the locative is used only for cities, towns, small islands, and a few special nouns that don't fall into those categories. For everything else you would just use in + (ablative form of the noun).