VCC sets the voltage

Nothing between VCC and your pin

When the spring touches the rod, it joins the switch's two legs, which on this board are VCC and SIGNAL. Nothing on the block changes the level on the way through: no resistor, no diode. So during a touch, SIGNAL is VCC. Put 5 V on VCC and your pin gets 5 V. Put 3.3 V on it and your pin gets 3.3 V.

3V3 beside a 3.3 V board

An ESP32, an ESP32-S3 and a Raspberry Pi Pico run their pins at 3.3 V, and none of them is built to take 5 V. Pick 5V in the figure with one of them selected: it reads HIGH, the knock sensor works, and the pin is past its rating several times on every knock. That damage is slow and it does not announce itself, which is why the wiring is the rule rather than a suggestion.

Connect VCC to the board's 3V3 pin. The block works exactly the same from it. Only the red LED is dimmer, and the capacitor takes a little longer to fall below the LOW line, as the last article showed.

5V beside an Uno

The Uno's pins run at 5 V, so its 5V pin is the right supply. 3V3 would work too, since the ATmega328P reads anything above 3.0 V as HIGH, but that leaves 0.3 V of margin to gain nothing.

VCC left off

With VCC unconnected, the switch has nothing to join SIGNAL to. Knock as hard as you like: the pull-down holds SIGNAL at 0 V and your pin reads LOW. The red LED does not flash either, which is the quickest way to tell this apart from a sketch problem.