It just my guess based how youtube shorts pick and show some of my video on their feeds....
1. Time & competitors... choose time when there're many viewers but not too many competitors post their videos
2. Topic... choose topic that kind of trending right now
3. Thumbnail... I think we can't really choose the thumbnail in shorts, but if the scene that appears on your video thumbnail interesting it may increase the chance to be picked.
Once youtube promote your video in their feeds, it will be there for short while... but once the audience start to stop watching or sliding their hands.........bye bye the promotion will end soon.
PS: the most view I can get from youtube shorts was 11,600 views