We can see from the picture that the booting process starts from the MBR (Master Boot Record), but actually, the booting process begins with the BIOS and continues from there. It’s the BIOS’s job to tell the computer which hard drives are available and to try to search for an active MBR record in the primary hard drive. The MBR record then tells the booting process where the /boot partition is located. Afterwards, the /boot partition is read and examined for any boot loader, which in our case is GRUB.
At that point, the booting of a computer is handed to the GRUB boot loader, which loads the grub.conf file and the kernel of the Linux system and then mounts the root partition, which in this case is /dev/sda3. The execution continues with the kernel image located on the /boot partition. The kernel is loaded and the execution is handed over to the Gentoo init process (with a PID number 1), which is already located on the system partition and is responsible for starting the entire system.