The video ratio for YouTube Shorts cannot be wider than a square unless you use additional video editing apps to fit the horizontal video into the 9:16 screen with black background bars. This way your video can be posted in wider formats like 16:9, 5:4, etc.  Another alternative to posting horizontal videos as YouTube Shorts is to rotate them before posting them in a YouTube Shorts format. A rotated video is technically the same as a vertically recorded video for the YouTube algorithm and all your viewers have to do is rotate their screen physically to watch the YouTube Shorts video in a higher quality.