How to create a short on YouTube (in 10 minutes)

[Right]

If you got a published recording open it in your browser.
Watch until you got the timestamp that you want as a short.
Open the YouTube app on your phone.
Goto history.
Open the the recording.
Click remix.
Goto timestamp and follow the steps.
Upload it.
It should be visible in YouTube Studio as Private.
Edit and publish.

This does not work with recording from others.