What factors determine where a population can live

The economy, supply and demand of the specific area, taxes, and you should also consider (if its a country) the currency, is it a tourist site? And look at the exports and imports, because if there are more imports in a country then prices will go up and people wouldn't like that, but if there are more exports then that means the country is getting more money, then more jobs, then more people, and etc.

The theory is land mass basically what they said.