The Popup can use any View as its Content and can be modal or not. It can be positioned on any side or corner relative to the Window, the Popup's parent or a View that serves as an anchor.

Note that it can be useful to define a Popup within the code beforehand & then display it when needed.

For xaml definition, a PopupContainer can be used.

In the following example, a Popup is created & then opened when a Button is clicked. The opened Popup is positioned at the bottom right of the sender, which in this case is said Button. The Popup's Content is made of a Border containing a Label that displays some text.

Visually, the code below would display this control:

private void Button_Clicked( object sender, EventArgs e )
{
  var contentBorder = new Border()
  {
    Background = Colors.LightGray,
    BorderBrush = Colors.Orange,
    BorderThickness = 3,
    Content = new Label() { Text = "My Popup...", TextColor = Colors.Red }
  };

  var popup = new Popup()
  {
    Content = contentBorder,
    IsModal = false,
    Anchor = sender as View,
    HorizontalOptions = LayoutOptions.End,
    VerticalOptions = LayoutOptions.End
  };

  popup.Open();
}

System.Object
   Microsoft.Maui.Controls.BindableObject
      Microsoft.Maui.Controls.Element
         Microsoft.Maui.Controls.NavigableElement
            Microsoft.Maui.Controls.VisualElement
               Microsoft.Maui.Controls.View
                  Xceed.Maui.Toolkit.Popup

Target Platforms: Windows 11, Windows 10, Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

Reference

Popup Members
Xceed.Maui.Toolkit Namespace