for shorts they have to be filmed or edited in a 9x16 and less than 60 secs (and have #shorts in the title or uploaded as a short) u can use a 16x9 video but u must reframe it into a 9x16 format. hope that help. 1 more thing when uploading a short, the ones that are 15 secs or less do better cuz its all about watch time. and if u upload a 60 sec clip youtube usually adds 1 second to be aware of that