Android debugger. Add your user to adbusers, get
android-tools and you are all set. On the device you need
to enable USB debugging from developer settings - tap builder number ~9
times. Once running, you can run a shell, move files, reboot, sideload,
etc..
A mode, akin to BIOS. Interact with it using fastboot,
enter it with adb reboot bootloader or by holding
Power + VolDown. You can flash the system from there,
including recovery.
Like a recovery USB stick, but inbuilt. Enter it form the bootloader
or adb reboot recovery. Use it for backups, flashing, shell
access, etc..
adb install foo.apk - install an apk onto the
device.adb backup/restore --twrp <file> - twrp-style
backups.