If a vehicle has the right turn signal on, but the driver signaled their intention to turn left with their arm, which signal prevails?

Whenever you want to warn of your intention to perform a maneuver, you can use acoustic, light or arm signals to warn other road users, provided the signals can be seen by the others. Signals made with the arm will prevail over acoustic and light signals, and will override any contradictory optical signal. Therefore, in this case, despite the turn signal light being turned on, the arm signal will be the one that prevails.