Technically “brightness” is what’s otherwise referred to as “Black balance” or “pedestal”, it’s taking all the pixels and adding an amount of luminance to them, hence the term black balance, because too much of it turns the blacks “light”, or
“contrast” is what increases the exposure of the image, it’s adding a multiplier to the luminance values which means the true blacks stay 0 but the whites move away from it, hence contrast.
OLED’s confuse this a bit as a black pixel is black regardless, so brightness and contrast become linked. When you adjust brightness in the TV settings what it actually does is muck about with the LUT (look-up-table) which is a map to tell it if it’s being told x,y,z for RGB then it looks those values up and displays the corrected value. Which in turn is shifting the histogram around to either allow more or less dynamic range in the shadows.
And then TV’s confuse this further because LCD+LED TV’s (i.e. most of them) try to mimic OLED’s, and oddly Sony have now gone back to LCD+LED’s for their production monitors which means you’re watching an LCD, mimic an OLED mimic a CRT.
Social media platforms, phone screens etc will also apply their own filters regardless, so if you want to have your photos displayed as close as intended you need to account for that. In Lightroom / ACDsee etc you can create presets which might correspond to different media, e.g. a postcard sized print will have far less sharpening than an A0 print, or printing on to canvas typically needs different pedestal (brightness / black balance) as the material soaks in ink and looks like it’s turned up anyway.
