Explain the relationship between a county and a county seat.

County - It is a geographical region of a country used for administrative or other purposes.

County Seat - A county seat is a administrative center, seat of government or capital city of a country.

So the relationship between both is that they both are used for Administrative purposes.

From~Charminglex

powers  are  denied  to the  national governmentin three distinct ways: some  powers, such as the  power  to levy duties on exports or prohibit the freedom of religion, speech, press, or assembly, are expressly  denied  to the  national government  in the  constitution.