Get the position of the button by passing values from first page to other using query string on click.
navigationservice.navigate(new Uri("/xyz.xaml?pos="+char,UriKind.Relative));
get the value to other page using onnavigatedto .
char = int.Parse(NavigationContext.QueryString["char"]);
Then place the video in a array and call the media element
string[] videos= new string[]{1.mp4,2.mp4};
then add the source of media element