How to root it?
First thing you need to do is unlock your bootloader.
You will need this on your PC:
https://developer.android.com/studio/releases/platform-tools.html
Full instructions for unlocking the bootloader, directly from the android site:
https://developers.google.com/android/images
Step 2 would be to grab the latest TWRP image:
https://dl.twrp.me/taimen/
I believe you'd want: twrp-3.2.3-1-taimen.img
And for a permanent install you'd need to also download the corresponding zip. Personally, especially with your current knowledge of the process I would skip the permanent install for now.
Next you need to also grab the latest stable version of magisk from here:
https://forum.xda-developers.com/apps/magisk/official-magisk-v7-universal-systemless-t3473445
Place the zip on your phone's internal storage.
Next boot into your bootloader either manually or using adb command:
adb reboot bootloader
Once you're there, use fastboot to boot the TWRP image:
fastboot boot twrp-3.2.3-1-taimen.img
That should load you directly into TWRP. From there install file, and use the magisk.zip you placed on your phone.
Reboot the device and you should be rooted.
Hopefully you understand how to use adb and fastboot, if not I won't be available for most of the day to answer questions but I'll check in later when I can and see how you're doing.