Si te refieres a "el corazón" del estado de Sao Paulo, entonces sí, es "the heartland of Sao Paulo State". Si te refieres al centro de la ciudad de Sao Paulo, entonces es "the downtown", si te refieres al "interior del estado de Sao Paulo", entonces es "the interior [region] of Sao Paulo State". No sé el portugues ... lo siento.

You could, but people from the US will misunderstand you if you do. "Heartland" refers to a regions that have small towns, no big cities. Definitely no tall buildings. It is likely near farms. There's no nightlife. There are no tourist attractions. There are almost no large businesses. It is nice, pleasant, and pretty but it is away from the speed of city life. Based on this article: https://pt.wikipedia.org/wiki/Interior_de_São_Paulo - the "Interior de Sao Paulo" is pretty much the opposite of that. It has a huge, growing economy. It has cutting edge companies. It has big cities. It has cultural growth. (As a comparison - Wyoming (which is what Americans picture when you say "Heartland") and Sao Paulo are about the same size. More people live in the city of Sorocaba than live in the entire state of Wyoming). Based on what I'm reading, I'd use the term "metropolitan area" or "region" or possibly "outer Sao Paulo."

Yes, you could call the central part of the state of São Paulo its "heartland". But the central part of the *city* of São Paulo would be "downtown São Paulo". If you want to refer to the part of the state that's far from the coast, you can say "inland São Paulo".