First let me start out by letting you know that this is purely for my own benefit. I hate having to lookup things I set once and rarely use again so if this doesn't work for you or you find errors, understand this is what I did to get things to work on my machines and YMMV.
Dimming a display. I find I use this on laptops at night when working around the family, also it helps with eye strain.
First find out the name of the display
xrandr -q | grep ' connected' | head -n 1 | cut -d ' ' -f1
The output on my MBP 2015 looks like this: DP-2. So, you can see where I use that information in the command below. Brightness can be a level from 0 to 10
xrandr --output DP-2 --brightness 0.7
Multiple displays & Rotation
To be continued...