If you want to play a YouTube video within the app, you can use the Video control (which you can find in the <Media> tab or <Insert> tab in the ribbon), and then set its Media property to the YouTube URL. Let's go with an example to see how this could be done.
Imagine that you have the following excel table.

Now, in the first screen, you would have a gallery. In my example, I have a custom vertical gallery, with three text boxes and one right-arrow shape. The text boxes are bound to show the text from the excel table, and the right arrow has an OnSelect property set to the second screen (Navigate(Screen2, Fade))

In the second screen, I added a Video control as mentioned in the beginning of this answer (and some additional controls with more information). Now, when you select the item in the gallery, it will navigate to the second page, and since the selected gallery item is bound to the video control, anytime you choose a different item in the gallery, a new video will be displayed in your app.
