1.BIOS: Basic Input Output System

When Linux is booted, BIOS starts up and performs the Power-On Self Test
Then POST checks the hardware operability of the components i.e. 
any hardware pieces such as / hard disk or SSD, keyboard, RAM and USB ports.and can be located through a direct installation on USB or DVD on Linux

When the POST process is complete and there are no problems, the BIOS will check the MBR for information about the bootloader and disk partitioning.There are 3 main types of bootloaders in Linux
LILO
GRUB
GRUB2

LILO & GRUB: They have become excluded due to the passage of time

GRUB2: is the latest basic bootloader in modern Linux distributions
GRUB2 stands for GRand Unified Bootloader version 2.Once the BIOS identifies the grub2 bootloader, it executes it and loads it into main memory (RAM).