Is it possible to add new scenarios and therefore new scenes from a remote location without having to rebuild the app to integrate the new scenes and scene modifications?

I don't know if you can have external scenes, but depending on how your scenarios are built you may have just one scene for all levels, and generate the level on the fly when you start one.

You can invent a text file format describing the level. At least that's what I do. One Menu scene and one Game scene (for all 80 levels).

Yes! Check out Asset Bundles and Addressables. This allows you to build certain aspects of your project separate from the main executable. Then you can just set up your main project to load whatever external asset bundles it can find dynamically.

So all you would need to do is build your base application. Then you can build any scenes you want as asset bundles and your base application should be able to find them and load them dynamically. We are currently using this system in our project, where different users will have access to different scenes. We just bundle up the scenes for each user, and our base application them. If the user wants more scenes in the future, then we just make the scene, bundle it up, and copy the bundle to their executable directory and voila, they have the new scene with no need for a rebuild of the base project.