Special boot modes

Recovery mode

fastboot mode

part of Android bootloader for flashing partitions should you wish to fix or modify filesystem

use a tool. on macOS and Linux: bundled with android-tools. on Windows: follow the ADB guide

https://developer.android.com/studio/run/win-usb
download, install inf file, if missed this step open devmgmt.msc, select device with exclaimation mark, install driver, browse manually, select file

see https://wiki.gentoo.org/wiki/Android/Fastboot for full list of commands

on leo: cannot boot into this mode without voiding recovery mode

EDL mode

Qualcomm Emergency Download mode, commonly known as EDL mode, is a special boot interface implemented on devices with Qualcomm chipsets. It lets you do special operations on the phone, such as unlocking the bootloader, read and write firmwares on the phone's filesystem or recover from being a dead paperweight. Unlike bootloader or Fastboot mode, system files needed by the EDL mode resides on a separate 'primary bootloader' that aren't affected by software modifications.

Booting into this mode, the phone's screen will briefly show the 'enabled by KaiOS' logo, then turn almost black as if it is off, but in fact it's still listening to commands over Qualcomm's proprietary protocol called Sahara (or Firehose on newer devices). With a [suitable digitally-signed programmer in MBN/ELF file format] and some programs, the most popular one being QFIL (Qualcomm Flash Image Loader), one can send commands from a computer to the phone over USB.