The add-on runs in background "main.js" only for youtube.com as can be seen from the manifest.
The script in background does listen for "mouseover" actions, and if the tagname "ytd-rich-section-renderer" is present on the document (it could also be reloaded by youtube) then it is removed, simple as that.
The source code will be published on my github at the following url: https://github.com/tin-z/Youtube-shorts remove

- License: MIT License
- Weekly downloads: 2
- Average daily users: 11
- Rating: 0/5 of 0 ratings
- Created: 2022-11-08 15:00:13
- Last updated: 2022-11-09 10:45:18
- Homepage:
- Support site and email
- Orig: https://addons.mozilla.org/en-US/firefox/addon/youtube-shorts-remove/
- API: {31103188-d0a8-4dfb-b3c3-17dfd052c85b}