Once Pi OS has booted, in Linux it appears to the operating system as a normal key input providing KEY_POWER scan code.
/etc/systemctl/logind.conf can be used to define what happens when the button is pressed. On the Pi OS lite image it'll shut down by default. On the full desktop image the shutdown dialogue will be displayed, and if you press it again, it will shut down.
The power button is also broken out to an unpopulated jumper header on the board, J2, near the J5 where the Real Time Clock (RTC) battery connector.
