where is nDisplayLauncher.exe ?

As of Unreal 5, there is currently no such launcher available. Instead, you can install and use a switchboard.

If you set the switchboard and turn it on and off, the icon will disappear.

Then unreal->Engine->plugins? Runtime-> switchboard or whatever, there is switchboard.bat after that path, you can run it with administrator privileges.

Regarding switchboard ndisplay, you can find the video.