✅ How does the button know what to do?

        <TextBlock Text="{Binding DisplayText}" Background="LightGray" Margin="10"/>          
        <StackPanel Grid.Row="1" Orientation="Horizontal" HorizontalAlignment="Center">
            <TextBox Width="550" Height="30" Margin="10, 0,0,0" Text="{Binding DisplayText}"/>  
            <Button Content="Set" Width="200" Height="30" />
        </StackPanel>

    public partial class MainWindow : Window {
        public MainWindow() {
            InitializeComponent();
            MainWindow_ViewModel vm = new MainWindow_ViewModel();
            DataContext = vm;
        }
    }

    class MainWindow_ViewModel
    {
        private string displayText;
        public string DisplayText {
            get { return displayText; }
            set { displayText = value; }
        }
    }