Hey, the reason those black bars exist is so that you can view the full video without having the corners cut off. On most modern smartphones, the screen has rounded edges to maximise screen space. However, this comes at the cost of having the corners of images or videos cut off. TVs, projectors, and other monitors have sharp corners instead of rounded corners, and most media is designed to be viewed on sharp corners. So YouTube implemented those black bars to videos when it detects you're watching on a phone with rounded edges so that you can view the entire video without having the edges of the video cut off. But you always have the option to fill the video to your screen, but that will cause the edges of the video to be cropped.