I made a little search about building a Safari app extension, and how adblockers do.
And to do it you have to build an xcode application,
this application will be displayed in your Application Folder with other desktop Apps.
Ok there is no interface, but it’s like a desktop app.
If you open it, it open a desktop window and you can have a link to safari or have some parameters about the safari extension.
From the Apple Developers Documentation
But at the end this is just a Desktop Application, with only the Safari extension part.
Don’t hesitate to upvote this thread => Desktop Application