Thanks to both of you.
I can successfully use Makedir(File.Dirrootexternal,"mydata") to create a folder on Android 11 device with TargetSDKversion set as 28, but not with TargetSDKversion set at 29 or 30.
I've tried using GetSafeDirDefaultExternal but the device will not display the Android/data/... folder that was created.
I'll have another look at ContentChooser to see if I can understand it better.
Does anyone know what Android Appstore require as the TargetSDKversion ?