The sliders on the dash work by opening and closing doors in the heater box to mix the air to be cooler or warmer. then of course you have fan speed and the selector to choose between heat or defrost.
There is no valve unless the car has AC. If equipped, the valve works by vacuum (controlled by the push button switch) to shut when the AC is used so that no heated water is passing into the heater core. The heat only box (non AC) receives heated water through the heater core at all times.