Simply put, the thermostat determines whether the coolant should be cooled in the radiator or not. That is, it decides whether the coolant should circulate through the "small" or "large" circuit of the cooling system.
Which circuit it connects depends solely on the temperature of the coolant. Thus, the operating temperature remains approximately constant.
Functioning of the thermostat
The thermostat is switched by a box filled with special wax and a valve plate attached to it. When the coolant is heated, the wax melts and expands. Due to its increased volume, the valve plate is forced to open.
As the coolant temperature rises, the thermostat increasingly increases the flow of coolant to the radiator and simultaneously closes the "small" circuit.
If the coolant temperature drops below the desired operating temperature while driving, the thermostat spring pushes the valve plate in the "closed" direction and blocks the flow of coolant until it is sufficiently warm again.
Advice: If the engine "boils" during a trip due to a faulty thermostat, then only towing or dismantling the thermostat assembly on site will help. If you decide in favor of dismantling, then you should first wait until the coolant temperature drops.
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