This is actually proprietary information that Google doesn't make publicly available. We're able to request state/province/national results, but we don't have details on how the results returned are calculated.
We use the "UULE" parameter to tell Google which location to return results from for any given query. They offer the ability to choose a city, state, province, or country as the argument for that parameter. For a city, we know that they're returning results from the geographic center of the city specified. They've never publicly stated precisely which area they're using when a state/province/country is used as the region. We like to think of it as an "average" for the region.
But if you choose a state/province/country as the location for a Google Local search, the Google Local map will show the geographic center of the region. So if you choose USA, the Google Local map will show a region somewhere near Kansas. We don't know if that means that those results are being emulated from the geographic center of the country, or if they're simply using the geographic center of the country for the graphical map display.
We like to caution customers when using state/province/country based locations, because those locations would never be used to provide a result in the real world. Whenever an individual searcher types a keyword into Google, they can ALWAYS determine the person's location down to at the least the city level - often nowadays, it's the neighborhood, street, or even postal address level. This applies even if someone is searching using Incognito mode or searching from a browser with a clear cache, since the location is determined via the IP address. Geographic location is always a ranking factor in real world searches, but that geographic location would always be more granular in the real world than a state/province/country.
So for customers who have clients that demand a state/province/national ranking, we're happy to pull that data from Google, with the caveat that it should at best be seen as an "average" result, and that no one outside of Google's internal search department truly knows what those results mean.