How do I stop my MacBook screen from dimming in the sun?

Turn off the two settings doing the dimming. In System Settings > Displays, switch off automatic brightness and True Tone. That stops your Mac chasing the ambient light every time the sun shifts on the deck. Keep it off Low Power Mode in Battery, which caps brightness to save juice. Give it shade or airflow in direct heat, since a hot panel throttles itself. If you also want it brighter than the everyday cap, MacBrightness opens the reserve on a MacBook Pro 14"/16" or Pro Display XDR. That's about 1,600 nits on the keys you already press, $5 one-time. The free Splitscreen Mode shows you the brighter screen first.