To create the DIY remote shutter button, [NirL] used a spare limit switch, resistor, and cannibalized an old set of earbuds for the cable and 4-conductor 3.5 mm audio plug. Most phones and camera apps trigger the shutter when they receive a Vol+ signal through the audio plug, which is done by connecting MIC and GND through a 240 Ohm resistor.

https://www.codrey.com/learn/selfie-stick-little-secrets/

When the user presses the selfie stick button to take a picture, the smartphone recognizes the change in resistance between Ring 2 and the Sleeve of the 4-pole audio jack plug

Not sure but roughly 240Ω +/- 1% button resistance is called for imitating the volume up function. Note that the microphone segment of the headphone jack triggers a picture once the circuit is closed by short-circuiting the GND (G) and MIC (M) wires, provided the button resistor is in the correct resistance range (210-290Ω).

More details https://source.android.com/devices/accessories/headset/plug-headset-spec

Obviously, this trick works only for Android since Apple devices demand custom circuitry to enable such functionalities.