How signal, logic power, and motor power actually connect
The motor driver is the translator between two worlds: your MCU's low-current logic signals and the motor's high-current, often higher-voltage supply. Concretely, the driver board typically has three groups of connections: logic-side pins (VCC/logic power, ground, and PWM/direction control pins from the MCU), motor-side outputs (the two wires to each motor), and a separate high-current power input (from the battery, sized for the motor supply voltage). The MCU never touches motor current directly, it only ever sends a PWM duty cycle and a direction signal to the driver, and the driver does the actual current switching. Keeping this mental model explicit while wiring, 'MCU talks to driver, driver talks to motor', prevents the most common beginner mistake: connecting a motor directly to an MCU pin, which can destroy the pin or the whole board instantly because MCU GPIO pins are rated for milliamps, not the amps a motor draws.
A second near-universal beginner mistake is forgetting a shared ground: the MCU's ground and the motor driver's power ground must be connected together, even though they're on separate supplies (logic power from the MCU's regulator, motor power from the battery). Without a common ground reference, the PWM and direction signals the MCU sends have no consistent voltage reference on the driver's side, and behavior becomes erratic, motors that run at the wrong speed, don't respond to direction changes, or work intermittently. This one wire is invisible in a schematic diagram if you're not looking for it, and it's worth checking explicitly before powering anything on.