BackgroundImagePage.cs

public class BackgroundImagePage : ContentPage
    {
        public BackgroundImagePage() =>
            Content = new Grid // this is the outer grid 
            {
                Children = 
                {
                    // this image fills the outer grid and sits behind the 'original' content
                    new Image { Source = new Uri("https://tinyurl.com/msdevbg"), Aspect = Aspect.AspectFill },

                    new Grid // this is the 'original' content
                    {
                        Children =
                        {
                            new Button { Text = "Column 0", BackgroundColor = Color.Black.MultiplyAlpha(.5) }
                                .Size(120) .Center() .Col(0), 

                            new Button { Text = "Column 1", BackgroundColor = Color.Black.MultiplyAlpha(.5) }
                                .Size(240) .Center() .Col(1), 
                        }
                    }
                }
            };
    }