Android/Fastboot

Fastboot is a communication protocol used primarily with Android devices. It is implemented in a command-line interface tool of the same name and as a mode of the bootloader of Android devices. It is included with the Android SDK package used primarily to modify the flash filesystem via a USB connection from a host computer. It requires that the device be started in Fastboot mode. If the mode is enabled, it will accept a specific set of commands sent to it via USB using a command line. Fastboot allows to boot from a custom recovery image. Fastboot does not require USB debugging to be enabled on the device. Not all Android devices have fastboot enabled.

Android device manufacturers are allowed to choose if they want to implement fastboot or some other protocol.

Install Android SDK Platform-Tools
Android SDK Platform-Tools is a component for the Android SDK. It includes tools that interface with the Android platform, primarily adb and fastboot commands

Booting into fastboot
You can flash a device when it's in the fastboot bootloader mode.

To enter fastboot mode when a device is undergoing a cold boot proccess by press 'vol down' + 'power button' in 7000ms, this applies to all android devices regardless model to restart device in any state, for enter fastboot mode keep holding the combination after device is restarted from the key combo and release the keys when device has entering download mode

Unlock bootloader
You can flash a custom system only if the bootloader allows it. Note, though, that the bootloader is locked by default. You can unlock the bootloader, but doing so deletes user data for privacy reasons. After unlocking, all data on the device is erased, that is, both private app data and shared data accessible over USB (including photos and movies). Before attempting to unlock the bootloader, back up any important files on the device.

Unlock bootloader only once, and you can re-lock it anytime.

Unlocking recent devices
All Nexus and Pixel devices released since 2014 (starting with Nexus 6 and Nexus 9) have factory-reset protection and require a multistep process to unlock the bootloader.

Enable OEM unlocking on the device
In Settings, tap About phone, then tap Build number seven times.

When you see the message You are now a developer!, tap the back button. In Settings, tap System, then tap Developer options and enable OEM unlocking and USB debugging. (If OEM unlocking is disabled, connect to the internet so the device can check in at least once. If it remains disabled, your device might be SIM locked by your carrier and the bootloader can't be unlocked.) Reboot into the bootloader and use fastboot to unlock it.