Twitch is not full screen on the Samsung Galaxy S20 and I assume all other models that have the centred pinhole front-facing camera design. When the phone was originally released, the app actually did go full-screen over the pinhole, but just a few weeks later the Twitch app updated to no longer do that. Watching on mobile now means having a black bar over one side of the screen. The whole point of phones like these is the feeling of a true full-screen experience and the current Twitch app design ruins it.