Windows only displays its built-in monitor brightness controls on laptops. Historically this has been because desktop monitors tend to have physical brightness controls already (though some don't, thank you Apple) and based on my own personal experience some monitors have unreliable DDC/CI implementations.
But that was in the past. Presently the DDC/CI interface on all of my current monitors seems solid - so I want to get Windows to allow me to control monitor brightness using Windows' built-in brightness controls (from the Action Center sidebar, keyboard hotkeys, and the Settings app) instead of using random DDC/CI apps from the Internet.
All of my monitors fully support Windows' VCP Monitor Configuration API (verified by using a VCP API developer tool) so I assume there must be some configuration option or setting in Windows that tells it to show or hide the brightness controls, but where is it?
Update: Based on my reading of the Windows HDK documentation for hardware brightness controls, it requires close cooperation of Windows' own "monitor driver" (monitors need drivers?) and the GPU's driver (NVidia, AMD, Intel, etc) with Windows: i.e. Windows' built-in brightness sliders seemingly always go through the GPU driver, as the expectation is hardware GPUs have control over integrated displays, so it doesn't seem possible for third-party applications to hook into this process.