Looking for addtional informations about Shutdown-Fragment example

Hi,

I use a device tree fragment example for a shutdown button on my Orange Pi One.

/dts-v1/;
/plugin/;

/ {
	compatible = "allwinner,sun4i-a10", "allwinner,sun7i-a20", "allwinner,sun8i-h3", "allwinner,sun50i-a64", "allwinner,sun50i-h5";

	fragment@0 {
		target = <&pio>;
		__overlay__ {
			gpio_button_0: gpio_button_0 {
				pins = "PA20";
				function = "gpio_in";
				bias-pull-up;
			};
		};
	};

	fragment@1 {
		target-path = "/";
		__overlay__ {
			gpio-keys-user {
				compatible = "gpio-keys";
				pinctrl-names = "default";
				pinctrl-0 = <&gpio_button_0>;

				power_button {
					label = "Shutdown button";
					linux,code = <116>; /* KEY_POWER, see include/uapi/linux/input-event-codes.h */
					gpios = <&pio 0 20 1>;
				};
			};
		};
	};
};

And I try to understand how this fragment works. AS I understand it right now the fragment uses the existing infrastructure of the SW4. Which parts of the system (driver, etc.) handle the input of the push putton and where can I find some additional informations about it?