how can they make a remote control cars?

The controller has two variables (speed and direction) controlled by how hard you push the little joysticks, or pull the trigger and turn the wheel (depends on what controller you have), and sends the respective information to the car via radio waves. The car has a computer that reads the radio waves and "tells" the car to do what you tell it to.
References :

While the mechanics of how they operate can differ greatly between different toys, the basic principle is the same. All radio controlled toys have four main parts:
Transmitter - You hold the transmitter in your hands to control the toy. It sends Radio waves to the receiver.
Receiver - An antenna and circuit board inside the toy receives signals from the transmitter and activates motors inside the toy as commanded by the transmitter.
Motor(s) - Motors can turn wheels, steer the vehicle, operate propellers, etc.
Power source

The transmitter sends a control signal to the receiver using radio waves which then drives a motor, causing a specific action to occur. The motor in a car may cause the wheels to turn, while the motor in a plane may adjust the flaps. The power source is typically a rechargeable battery pack, but sometimes it's just normal batteries.

For a detailed description try bottom link
References :
http://www.howstuffworks.com/rc-toy.htm