A switch in the ground wire

The buzzer is always on the supply

The buzzer's + lead sits on VCC all the time, sounding or not. What is switched is the other end. Its − lead goes to the collector of an S8050 NPN transistor, the emitter goes to GND, and the base decides whether current can pass between them.

Press SIGNAL HIGH and watch it in order. The pin pushes a small current through the 1 kΩ into the base and out of the emitter. That small current lets a larger one flow from VCC down through the buzzer into the collector, and the buzzer's own oscillator starts. The LED, wired beside the buzzer, lights on the same path.

This is a low-side switch: the usual way for a microcontroller pin to turn on something that needs more current than it should give.

What the base needs

A transistor's base behaves like a diode to the emitter: it sits at about 0.7 V whatever you put on it, and the resistor in front of it sets the current. From a 3.3 V pin that is (3.3 − 0.7) V across 1 kΩ, about 2.6 mA. From an Uno's 5 V, about 4.3 mA.

The collector can carry up to a set multiple of that, the transistor's gain. The BOM orders the S8050's 200 to 350 grade, and the buzzer and the LED together want about 25 mA with VCC on 5 V. At the lowest gain, 200, that needs 0.13 mA of base current. The pin gives about twenty times as much.

So the transistor is not being asked to amplify. It is driven hard into saturation, fully on, and the datasheet's typical curves put only about 20 mV across it at this current. The buzzer gets almost all of VCC.

What the pin gives

About 2.6 mA from a 3.3 V pin, and it is the only current your board supplies. That is well inside what an ESP32, an ESP32-S3 or a Pico pin can source, and an Uno's 4.3 mA is well inside its. The buzzer's current comes from VCC and returns through GND. It never flows through the pin.

No pull-down

Many transistor boards put a second resistor from the base to GND, so a disconnected input reads as off. This one does not: SIGNAL reaches R1 and nothing else. With nothing driving SIGNAL the base floats, and a floating base with nothing pushing current into it usually stays off. Usually is the word. A pin that is pulled HIGH while your board starts up turns it on, and that decides which pin to wire it to.