The vast majority of the elements of the American system are “borrowed.” It would be very strange if a system of government could be made that did not borrow heavily from previous systems.
The most obvious aspect of American government that is borrowed is the basic idea of democracy. This is something that had been around at least since the days of the ancient Athenians thousands of years before the founding of the United States. The modern system of representative democracy had been in existence in England for centuries.
From the British, the Americans also got the idea of a bicameral legislature. The British had the House of Commons and the House of Lords, one of which was elected and one which was not. Both American houses were elected to some degree under the Constitution, but the idea of a bicameral legislature in which the upper house (Senate or House of Lords) was supposed to be taken from the upper classes came from the British.
The American system of separation of powers between three branches of government did not come from any system that was already in existence. However, you can say that it was borrowed as well. The American Founding Fathers did not make it up themselves. Instead, they got the idea from such Enlightenment thinkers as Montesquieu.
Thus, many important parts of the American system were borrowed from existing systems or from thinkers from outside the United States.