Where do the exact locations of the phone buttons combinations stored? Where do the exact location of the phone button combinations like power reset while holding Volume down + Power, booting to recovery by holding Volume up + Power + Home (if there is a Home button) is it on the system, system partition, boot partition, any other partition, etc.
It's not stored in the Android system, but somewhere in the phone's firmware that's not modifiable.

Such functionalities like hard poweroff or hard reset is controlled by the underlying hardware directly, not by Android system. Therefore the button combos are stored in the hardware controller and are usually not modifiable by software.