I think you are looking for the geotapped event on the map view. There a ton of good examples on this api. once you know what the geo elements layer is you can determine if it is a Shapefile or KML and then select,show callouts, etc..
<esriUI:MapView x:Name="mapView" GeoViewTapped="mapView_GeoViewTapped" />
then handle it and search the area tapped.
private async void mapView_GeoViewTapped(object sender, Esri.ArcGISRuntime.Xamarin.Forms.GeoViewInputEventArgs e)
{
var results = await mapView.IdentifyLayersAsync(e.Position, 15, false, 10);
foreach (var result in results)
{
var attributes = result.GeoElements.FirstOrDefault().Attributes