I think it might be helpful if you were to explain why you chose to cast sender, which is an AutoSuggestBox, to an AutoSuggestBox on line 48 of MainPage.xaml.cs. As far as I can see it was done only to make lines 49 & 50 (arguably) a little more readable. I rewrote your code as
sender.ItemsSource = selectionItems.Where(p => p.StartsWith(sender.Text)).ToArray();
which I understand might be daunting for an absolute beginner but I do think that explaining the difference between what you originally wrote and
var filtered = selectionItems.Where(p => p.StartsWith(sender.Text)).ToArray(); sender.ItemsSource = filtered;
would be immensely helpful.
Along those lines I think it would also be helpful if you were to explain a bit about selectionItems as that seems more magical than any other piece of code.
That being said, thank you so much(!) for this informative series. It's a fantastic beginner's guide!