Having followed the power button pushed, some events occur until a typical PC loads its operating system:
From Read Only Memory, the cpu loads the boot strap (ROM). The CPU is now about to continue implementing orders.
The machine conducts a self-test upon powering on (POST). If an error is found during the POST phase, the machine may display an error message and shut down. If nothing goes wrong, the machine can proceed to the next level.
The Basic Input/Output System (BIOS) or Unified Extensible Firmware Interface (UEFI) can perform a boot device sequence scan via a collection of storage devices. As the Master Boot Record code locates the first bootable storage device in the catalog, it searches for an active partition with boot sector code. This boot sector is referred to as the Volume Boot Record (VBR).
The VBR is then executed, which loads and executes the Operating System's kernel.
The Operating System is now in control of the hardware, and the machine is ready for operation.