Unlike TikTok and Reels, on YouTube the shape decides whether a video becomes a Short: a landscape video ends up as a regular video on the channel, not in the Shorts feed. For Shorts it really has to go portrait.
If you publish the same material as a regular YouTube video and as a Short, you need two versions: 16:9 for the channel and 9:16 for Shorts – from one video, with its own crop for portrait.