The US is special. It's a true federative nation, with a fairly weak federal government and strong individual states. So the president actually does not have a lot of power inside the country. He can't just fire the governor of Minnesota or unilaterally cut funds to a state that he doesn't like.

But the foreign policy is where the president's authority is outsized. So that's why Trump is so focused on it, it's one of the few areas that he can directly control.