Disable backlight and permanently show ambient display using root? I own a Rooted Nexus 5x on the Pixel^2 8.0.0 ROM, and when I saw a real Pixel 2 on its charger at night I noticed that it didn't have "backlight" on (black was actually black) and it permanently showed the ambient display.

I don't see a settings option for this, can I turn it on using root?
It's called Always On display and you ideally shouldn't be using it on your Nexus 5X because its an LCD screen, not AMOLED like Pixel. You can flash a different ROM which has this feature or if you have Android 8.1, it's already baked in all devices just needs to be manually activated. Follow this XDA article

You need to install Andromeda Substrantum on your phone. Download Pixel Enabler or Always-on-enabler subtrantum theme and install it.