How do i make the black bars on mobile emulators look like how most youtubers do it?

Here's an example of what I'm talking about https://www.youtube.com/watch?v=m8G66SWAELA . I've seen a bunch of youtubers do this when they make a video on a mobile game, pewdiepie, captainsauce,graystillplays. but I haven't been able to figure it out. How do I do it?