Guides/help/tips about making extension based app like vscode!

So I want to develop a software that will display
and alert user if changes happen in different sites .so that I don’t visit them until there is new update /,can set to show update after certain time , end goal is to avoid unnecessary content that end up taking more time ./ Mental space.

Why extension ?
Just have curiosity how they are developed to supported in a app plus it will make the app more fluid by being able to scrape /process the data in different ways for different sites

Like changing in scrapping different for

  1. Some will require login
    2.some api keys
    3 some have to be scrapped , etc