XCSoar uses whatever controller is provided by the OS (Android, Linux, ...)

True, however there are many external controllers that can be interfaced to the OS you mention. Many of the Android devices can support OTG devices like mice and keyboards as well as Bluetooth devices. I've used just about all those combinations.

I currently have XCSoar running on an OperVario system. I can plug in a mouse, keyboard, or a custom controller that I can mount on my control column. I have the latter.

MB