YouTube decides based on these factors if a video that is published is regarded as a Short or a “normal” long form video:
The video is less than 3 minutes in length
The video has an aspect ratio of 9:16 (portrait mode)
We don’t have direct control over this, YouTube decides this automatically.
If a video uploaded through our Content Publisher is being posted as a long form video, when you expected it to be a YouTube Short, please contact YouTube directly: https://support.google.com/youtube/answer/6249136