Seems this is not an easily accessible built-in feature of MacOS: [Wayback/Archive] macos change brightness of external monitor at DuckDuckGo
Basically there seem to be two apps that are most popular:
- Lunar
- MonitorControl
Lunar is free, but has a fee for some features. MonitorControl is free derived from Lunar, but has less features than the paid Lunar license has and is less often updated.
Both have been developed in Swift and are on GitHub:





