How do you anchor within Windows universal app - xaml

Question

I've been looking on Google and all over the place for the answer on this, but I'm coming up empty.
I'm trying to make a few windows on my Windows universal app software. I want it to be able to resize itself as the user makes the window bigger or smaller. I can't find anywhere on the properties where it says anchor, and I'm not finding any help with the auto complete with the .xaml.
Any help on this will be welcome
UPDATE for – Justin XL or anyone that can answer
Below is a image of what I'm seeing. I'm trying to get all 4 blocks to take equal space at any given time. Obviously that isn't what I'm getting.
Anyways, I hope you can take a quick look at the code below the picture to tell me what I'm doing wrong.
The code below is what I'm using. I'm not sure if I'm doing something wrong since this is my first Windows app.
<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="*"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<Grid Background="Gainsboro" Grid.Column="1" Width="Auto" Height="Auto">
<Button x:Name="Click_Me" Grid.Column="1" Content="Click Me" HorizontalAlignment="Center" VerticalAlignment="Center" Click="Click_Me_Click" Width="107" /></Grid>
<Grid Background="Gainsboro" Grid.Column="1" Grid.Row="1" Width="Auto" Height="Auto">
<TextBlock x:Name="textBlock" HorizontalAlignment="Center" TextWrapping="Wrap" Text="TextBlock" VerticalAlignment="Center" Grid.Row='1'/></Grid>
<Rectangle Fill="Gainsboro" Width="8" Margin="622,0,10,0" VerticalAlignment="Stretch"/>
<Grid Grid.Column="0">
<WebView Name="Video" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" Width="Auto" Height="Auto" Grid.Row="0" Grid.Column="0" />
</Grid>

Answer 1

First, you should remove HorizontalAlignment="Left" on your MySplitView to make the content fill the entire page.
Also, I would change your hard-coded Margin on the Rectangle to -
<Rectangle Fill="Gainsboro" Width="8" VerticalAlignment="Stretch" HorizontalAlignment="Right" Margin="0,0,-4,0" />
Note the -4px(right margin) here. It's because the Rectangle itself is 8px wide and aligned on the edge of the first cell in the Grid, so I shifted it to the right by 4px to make it absolutely sit in the middle of the page.
Hope this helps!