Step by step commands to update your android device to the latest version of Android supported, installing TWRP, Magisk and Kali NetHunter especially on a Nexus 6p on Android version 8.1 (oreo).
brew update
brew upgrade
brew install android-sdk
brew install android-platform-tools
adb reboot bootloader
Download https://dl.google.com/dl/android/aosp/angler-opm7.181205.001-factory-b75ce068.zip
cd ~/Downloads/
unzip angler-opm7.181205.001-factory-b75ce068.zip
cd angler-opm7.181205.001-factory-b75ce068/
sudo ./flash-all.sh
adb reboot bootloader
Download https://dl.twrp.me/angler/twrp-3.5.1_9-0-angler.img.html
cd ~/Downloads/
adb reboot bootloader
mv twrp-3.5.1_9-0-angler.img twrp.img
fastboot flash recovery twrp-3.5.1_9-0-angler.img
fastboot reboot
In Android Browser Download the Magisk Manager App: https://tweakdroid.com/downloadlinks/magisk-manager-app/
If needed follow prompt to allow installs from Chrome Downloads
Then open Magisk App
Under the Image window click install, select the img for your device
adb reboot bootloader
Click volume down twice then top side button to boot to recovery
The swipe to write
Then click install and select the magisk.zip in your devices Downloads folder
Click Reboot
Update Magisk and TWRP via their apps
On Android Device download: https://kali.download/nethunter-images/kali-2021.3/nethunter-2021.3-angler-oreo-kalifs-full.zip
adb reboot bootloader
Click volume down twice then top side button to boot to recovery
Click install, select Kali Nethunter, swipe to install
Click Reboot System
Open the NetHunter App, click allow and grant forever a bunch of times
Click the menu button in the NetHunter App, then Kali Chroot Manager, click Start or Instal Kali Chroot
Click the menu button Click the menu button in the NetHunter App, then Kali Chroot Manager, Add MetaPackage, Select Large
Open a Kali Terminal
apt-get update
apt-get upgrade
apt-get dist-upgrade
apt autoremove
And ENJOY!