Stick Layout in Xamarin Forms to bottom

You can use VerticalOptions to send layout to bottom.

var stacklayout = new stackLayout()
{
     VerticalOptions = LayoutOptions.EndAndExpand
     Children = {
      //your elements
     }
}

<StackLayout>
  <StackLayout Orientation="Horizontal" VerticalOptions="Start">
    <!-- top controls -->
  </StackLayout>

  <StackLayout VerticalOptions="CenterAndExpand">
    <!-- middle controls -->
  </StackLayout>

  <StackLayout Orientation="Horizontal" VerticalOptions="End">
    <!-- bottom controls -->
  </StackLayout>
</StackLayout>

Make sure to have no more than one child with Expand options for best performance.