Verify your brightness settings. Browsing and typing in word processors with often light background would stay comfortable with lower brightness than watching a movie or TV show.
Apple TV+, and other streaming video content in the TV app, should automatically adapt to the screen’s HDR capabilities, and look its best.
MacBook Pro screens may dim significantly when not connected to AC power, when only on battery. Try externally powered.
… and some of the suggestions don't correspond to the menu items on my new Mac
Either those suggestions applied to earlier OS versions, or different models Mac, or different devices all together.
Can you specify the suggestions?
Else: If your Mac display is dark - Apple Support