BOOTLOADER MODE: Phone shut down. Press and hold the volume down and power buttons at the same time. When the Google splash screen appears, release them.
Accessing the Bootloader with ADB:

adb reboot bootloader

RECOVERY MODE: Accessing Recovery with ADB

adb reboot recovery

FASTBOOT MODE: Fastboot is a diagnostic tool. It’s essential if your phone fails to boot for some reason. It’s most commonly used to install a custom recovery. Fastboot are utilities that unlock access. Fastboot works when Android is not running, and the device is booted into “Fastboot mode”. It enables you to access all your device’s partitions — not just the Android system, but also the data partition, the boot partition, and so on.
See this link: https://forum.xda-developers.com/p9/how-to/huawei-p9-brick-easiest-to-unbrick-t3662348/page2#post77267214

TWRP storage 2 power, letras amarillas, 2 abajo

RECOVERY MODE ERROR:
Func NO: 11 (recovery image) Error NO: 2 (load failed!) PRA-LX1

Maybe a solution: needs x3 buttons pressed:
https://mobiletech360.co/huawei-p8-lite-2017-pra-lx1-stock-firmware-rom-android-8-oreo/

• keep pressing at the same time all three hardware buttons ( volume +, volume - and power ) … your device will start automaticly to install UPDATE.APP - if UPDATE.APP is compatible 100% with your device !

OR

• extract it and put dload folder on the root of an sdcard (sd card should be 8bor more).
• Insert sdcard into tye phone and on phone powered off hold volume up + volume down+ power.
• When you see huawei logo let go of the power button but keep holdiing volume up + volume down until you see emui logo

BUT update.app fails after 5 seconds, I need to relock my bootloader:

• https://forum.xda-developers.com/p8lite/help/error-mode-func-11-recovery-image-error-t3801756

fastboot oem relock 0125197374635555