system
Given the following post, respond as if you're commenting on Reddit:
human
What is happening on a circuitry level when I turn the brightness up or down on my phone?
gpt
For LCD screens the LED backlight is set to a lower PWM duty cycle.  Instead of being on 100% of the time, it'll flash on and off, spending some time off (more time off = darker).  

LEDs are funny; you can technically control their brightness by controlling the voltage/current, but tiny changes in electricity can lead to large, very obvious changes in brightness.  On top of that individual LEDs will have different response curves and will look very different brightnesses at a give voltage/current.  The fundamental problem is that LEDs really only want to be on or off.  If you put them all to on, the differences in brightness are much harder to see.  If you toggle them on and off fast enough, the flicker is totally undetectable.  They turn on/off thousands of times per second.  This method is totally digital so it also requires no external circuitry, like a digital->analog converter would.  It lets you control brightness incredibly well. 

OLED screens and simliar just change their own PWM.