System = Micro controller [or] Micro processor [or] some circuit to do required task for human. GPIO -> Input mode, Output mode. Keyboard, display
When you power on your system, voltage will be applied to your circuit and then to controller/processor. How voltage supply propagates and the sequence in which peripherals get voltage on circuit board called power sequencing. Once votage applied to controller/processor its initial ROM code will start executing to find and load its authenticated firmware for the processor/controller initializations, that includes peripherals, IO, and memory.
Boot loader UEFI Firmware Code boot firmware
Linux boot phases