Fun fact! Many OLED displays dim the whole image not by making the diodes dimmer, but by pulsing the diodes fast enough to match the desired brightness of each pixel.
You can test this by taking your OLED phone, pulling up an image, and then waving it around at different brightness levels; the observed image would become blurry at high brightness levels, but would separate into distinct “frames” at lower brightness.
I’m not sure if every OLED does this; just from the phones that I have used.