How Do You Turn (Corner) Maintaining Near 100Mph

This depends on alot of factors.  I never let off the gas during a race or drift event, the reason for that is I keep the engines rmps at the max all the time.  The reason people can take a turn at 100 mph depends on the car the set up and the apex they use to enter and exit the turns.  I only use the handbrake during races and cop pursuits and only tap the handbrake otherwise you will spin the car.  Drift events are another story handbrake is all I use