Why is the background of the window and keyboard black?

I run the immersive app rendered by Metal (here is code), and found that the background of the window and keyboard black is black, as picture shown. I have several questions about it.

1. Can you explain why it is happening in terms of rendering mechanism?
2. Can the background be configured in other colors, such as semi-transparent, white?
3. Do you recommend using Window scene in fully immersive application using Compositor Service?

thanks.