Android Bootloader is a small piece of code that runs when the device starts up. It checks for firmware updates and loads them if present, then passes control to Android OS. If there’s no update available, it asks user whether they want to boot into system or recovery mode.
Why does one need to unlock bootloader?
Bootloaders contain code that checks for firmware updates. If the device is unlocked, its bootloader can download and install whatever update is available without user intervention.
Bootloaders are often locked to prevent unauthorized access, but there are security benefits as well: a locked bootloader makes it impossible for an attacker who gains physical control of your phone to load anything other than what you’ve installed yourself. This means no malicious code could be introduced into the system by connecting your phone up to a computer or downloading malware from the Internet onto it.
A booted Android OS image consists of multiple partitions (typically one per major architecture such as armv-a) which have read/write permissions on them individually an read-only permissions on them collectively.