The coolant lines Tee off from the engine and run through the defroster and all the heaters and then back on a return line. On my bus, it was a 1" diameter hose. Each heater has the line go back and forth inside it as a sort of heat sink, and will generate heat whenever coolant is running through the lines. Additionally, the defroster and each heater has a 2 or 3-speed fan that will greatly increase the heat output of the heaters.
There is typically a lever-operated heater valve near the driver to shut off the heater lines. Additionally, most buses have manual shut-off valves located near the engine for maintenance purposes. As the lever-operated valves are unreliable, often the manual valves are closed during summer months.
Additionally, full length buses will have a "heater booster" switch. This turns on a 12 volt water pump to increase coolant flow to the heaters. Without it turned on, coolant flow will be very low and very little heat will be generated.
Some diesel engines also have a thermostat that will prevent heat from going to the heaters until the engine is fully warm, but I don't know how many buses have those installed.
Often, there will also be a lever-operated vent to let outside air in through the defroster. If it's open, you'll have a hard time keeping your bus a comfortable temperature in hot or cold weather.