Explanation
That last point is the whole format. Long-form video is served to people who searched for it or subscribed to it; short-form is pushed at strangers mid-scroll. Everything characteristic about the format follows from having to earn a viewer who had no intention of giving you one: the front-loaded hook, the absence of preamble, the on-screen text, the vertical crop.
It also explains why the format rewards volume in a way that other channels do not. Each post is an independent audition rather than an episode in a relationship, so a flop costs almost nothing and the correct response to one is another post rather than a retrospective.
Practically, the three surfaces are similar enough that one 9:16 file works on all of them, and different enough in cadence that the same file is worth posting to each.
The common mistake
Repurposing a landscape video by adding bars top and bottom. It signals that the video was made for somewhere else, which is a reason to scroll.