Based on this article: https://workspacevita.com/how-to-use-own-videos-screensavers-mac-catalina/
The difference is that the Aerial Configuration screens shown there are outdated.
First, you need to prepare the video file so it would be visible to Aerial. Due to security.
- Create
/Users/Shared/screensavers
folder. - Put a copy of your video file to
/Users/Shared/screensavers
. - That should be enough.