In your car, the coolant system is the pressurized system. So, whenever your car is cooling down the excess amount of coolant also cools down and returns to the radiator. Whenever you are driving the car, the coolant in the radiator can be warmed up and expands as well as flow into the reservoir tank. If you notice any issues in the coolant system, then find out the coolant leaks from the exhaust manifold. Go through the video to know about the root problems behind the coolant leaks from the exhaust manifold.
Source:https://www.mechanicsdirect.com/