There have been lot of products developed out of a Raspberry Pi ranging from Internet of things to FM transmitter. This particular product aims at controlling a commercially available RC car. Although the code along with the explanation on this project is available on the project website, I suggest you try out writing your own code if you are a beginner. This project also has implemented image processing in the initial stages to find out the correct frequency on which your RC car will work. Basically every car will have a different frequency and an available option is to brute force to find the correct radio frequency.
If the radio receiver is a complex one, you would have no option but to use an oscilloscope. The entire code has been written in python which can be clone from github and is customizable to be compatible with your car. Initially, the program will find the correct frequency by using image processing to determine at which frequency the car moved. The next step will be to determine the PWM or frequency values of the control signal (left, right etc.). Once that is completed you can execute a set of scripts to control your car via arrow keys.