[Google Pixel XL Questions & Answers] capaficionado:
Hey, I recently got my Pixel XL (27th, new for only £200!). I am telling you from what I had experienced. I believe I was shipped a Verizon phone, as I cannot unlock the bootloader and make calls as I am with Vodafone in the UK. I followed a guide on XDA and you should to if you have the greyed out "OEM Unlock" Setting in Developer Options: https://www.xda-developers.com/unlock-bootloader-verizon-google-pixel-xl/

So, if you have had any experience flashing and unlocking boot loaders with the Nexus devices, you will feel mostly at home. TWRP is very easy to flash, but with extra steps. However, remember that not all goes well for everyone, so I am not responsible if anything goes wrong, that doesn't mean that I wouldn't help you though. lol :D Any links that you may need are listed at the bottom.

Installing TWRP:

Firstly, download the twrp-pixel-installer zip file on your Pixel:

-Pixel devices: https://dl.twrp.me/sailfish/

-Pixel XL devices: https://dl.twrp.me/marlin/

You would want the latest zip file on your Pixel. Why Pixel? I have tried to flash the recovery from the PC, but I get a partition error, I believe this is normal and I think I was doing everything incorrectly at my first try. Instead of flashing the recovery the traditional way, we will temporarily boot into the recovery though fastboot.

On your computer, download the IMG file of twrp: "twrp-3.2.x.x-devicename.img". This is the image file we are going to use to boot into TWRP. Make sure you have the correct adb drivers from Google and the platform-tools itself and go into the bootloader screen on the Pixel. Then connect your Pixel and start CMD within the directory of which your platform-tools is in.

Type:

fastboot devices

to check if your device is recognised by the computer. If not, try reinstalling the Android Drivers, or try a different PC (which helped me sometimes).

After that, type

fastboot boot "directory-of-twrp-img"

Just drag-and-drop the img file in CMD. It will fill in the directory for you with the speech marks.

Your Pixel will boot into recovery, just swipe to allow modifications or set your language first if you desire. Then go into the install section and flash the twrp-pixel-installer zip file that you have downloaded for the first step of this guide. It will run the installer in both slot partitions (a & b) of the Pixel to make sure that all goes smoothly (i think).

TWRP is now installed on your system. :D

Installing Magisk:

Magisk is a universal systemless root that is designed to work on Android 5.0 and higher. This works on any phone and you do not need any specific zip file. Just download the Magisk Manager apk from XDA and install. Then go into the app and it should ask you if you want to download the latest Magisk zip file. Just download it and download as a zip file only. Then reboot into TWRP and install the zip file in the Downloads folder.

I hope my long reply has helped you here and if you have any questions, or issues, feel free to ask. Contact me on my Telegram if you wish: t.me/yomarshmellow

Links:

platform-tools: https://developer.android.com/studio/releases/platform-tools

Google USB Driver: https://developer.android.com/studio/run/win-usb

Max Lee's video guide on this: https://www.youtube.com/watch?v=Hg5986wL-3k