The receiver converts that position information to move the steering to match that angle. The same thing happens on a gas car with another servo being controlled by a stick or trigger. The servo angle corresponds to the transmitter and causes a rod to open the throttle on the carburetor. On electric cars a servo can be used to connect an electrical circuit that controls the motor speed by dumping power through a resistor. This resistor lowers the voltage to the motor.
Some speed controls are actually wires wound around to form a rheostat or adjustable resistor.
This is like the types of controls found on slot cars or on dimmers for light bulbs. The next step up is called an electronic speed control. In this type of control the motor is actually cycled on and off very rapidly. When the motor is at full speed the battery is supplying it full voltage. The receiver still translates the stick or trigger information, but it is then related to what percentage of full voltage is supposed to be applied to the motor.