Besides normal booting of an android device, there are two more systems maintenance mode. The bootloader or fastboot mode and the recovery mode. We can get into both the modes via startup key combinations or by using adb commands.
What is a bootloader and how to get into the bootloader mode on your android device?
A bootloader is a computer program that loads an operating system (OS) or runtime environment for the computer after completion of the self-tests.
Bootloader is like BIOS to your computer. It is the first thing that runs when you boot up your Android device. It packages the instructions to boot operating system kernel. Basically, Android device has a storage space(disk) that has several partitions, which holds the Android system file in one and all the app data in another. Bootloader serves as a security checkpoint that is responsible for checking and initializing the hardware and starting software. You can get into this mode using adb command as well as by pressing device-specific buttons