What's the easiest way to adjust screen brightness on a Mac?

The fastest way is the brightness keys at the top of your keyboard: F2 to go brighter, F1 to dim. They're the same keys you already press, nothing to learn. On an external keyboard, hold Fn first; for finer steps, add Option-Shift. Two slower backups: open Control Center in the menu bar and drag the Display slider, or open System Settings, Displays, and drag Brightness. Both reach the same ceiling the keys do. Say you've maxed brightness on a MacBook Pro 14"/16" or Pro Display XDR but want more. MacBrightness opens the brighter range macOS holds in reserve on those same keys, $5 one-time, free to try.