Hello,
Welcome to Microsoft Q&A.
According to the results of our discussion, what you need is the "picture in picture" function provided by UWP, which can turn the current application window into a small window on top.
Usage
ApplicationView.GetForCurrentView().TryEnterViewModeAsync(ApplicationViewMode.CompactOverlay);
For obtaining HDMI-related information, you can refer to this document: HdmiDisplayInformation.
Thanks.