Windows Phone 8.1 is not same as Windows Phone 8.
Windows Phone 8.1 is not out yet. There are no leaks. There are only predictions. There are no feature releases or enlisting. Hence we don't know what Windows Phone 8.1 will have. It *may* have Cortana. How will it work, will it be an app, will it be in built? - we don't know. Anything further in this thread is just what everyone "thinks" it should be like or will be like. Nothing official.