What is the principle by which a cooling system(Radiator) in a motor car works:

A car radiator cools engine coolant by transferring heat from the hot liquid to the air flowing across the radiator surfaces — this is primarily convection: moving fluid (air and coolant) carries heat away from the radiator fins and tubes [1]. While heat must conduct through tube walls and fins from the coolant to the external surfaces, that conduction is an internal step; the dominant heat transfer from radiator to surroundings is convective heat exchange with air. Radiation contributes only a minor portion at typical operating temperatures. Thus, the working principle of an automobile radiator is mainly convection, supported by conduction within the solid components [1].

Sources

1. [1] https://www.uti.edu/blog/automotive/radiators