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1. Please explain me this line :
if(abs(angRL)>abs(angRR)) {angSpeed=-angRL;} else {angSpeed=angRR;}


This is a hack that's not quite as good as it could be.
What that function does is it tries to find the cars current speed (angular speed of its rear wheels). Since one of the wheels might be stuck in a corner with the other one spinning the if clause picks the larger velocity of the two wheels. For the sample car the wheels were mounted with the hubcaps facing outwards. This means that the right rear wheel will spin forward, but the left wheel is 180 degrees rotated with respect to the left wheel. Because of that rotation it returns it's forward speed as a negative value. That's why the angular velocity of the rear left wheel (angRL) is negated.
If your wheels are symmetrical you can just duplicate them without rotation and remove the minus sign in that line above.

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If you make handbraking when car speed is very high, car will stop changing orientation /I don't know how to call it/. In your tutorial car doesnot change it's orientation, it will just stop.

I think you are saying that your car spins to the left or right when you pull the handbrake. This comes from tiny differences in friction between the wheels. It can be solved by applying less braking torque or by applying the torque smoothly over time. If you look at the top of the file you should see the constant definition for braking torque which is about 50% of maximum torque delivered by the drivetrain if I remember correctly. To summarize: make your brakes less effective so that the wheels won't lock and the car won't spin.