Ignoring the oddball Chevy motors with reverse flow cooling, the water pump inlet is the lower radiator hose. Coolant flows rearward in the block and up into the head, then forward to the crossover at the front of the intake. If the t-stat is open, the coolant flows from there to the top of the radiator. When the t-stat is closed, the pump simply circulates the coolant already in the block in the same manner. The fact that the lower radiator hose is open to the pump doesn't mean that there is any flow in it. The closed t-stat prevents any flow in the radiator, so there is no flow out of the radiator into the pump either.