Why aren't DC motors getting enough power?
Your breadboard diagram shows the 9V battery shorted. That is, its terminals are connected by nothing but a wire (internal to the breadboard). This doesn't accomplish anything except make the battery hot. There's no way for the current from the battery to go anywhere except through this short wire. Notably, there's no way it will go through the motors.

The only power source for anything on the breadboard is coming from the GND and +5V connections on the Arduino, which are not sufficient to power the motors, because they are not designed to supply much current.

You need a voltmeter to do electronics. Trying to troubleshoot a circuit without a voltmeter is like trying to repair a car with your eyes shut. It's impossible to say exactly what is wrong with your circuit if all you can know about it is "it doesn't work".