How telegram web handles backend?

I have recently been scrolling through telegram web GitHub page. In one of it's part of readme.md and it is written under technical details:

The app is based on the AngularJS JavaScript framework, and written in pure JavaScript. jQuery is used for DOM manipulations, and Bootstrap as the CSS-framework.

And I am wondering how is it possible to write such a complex and dynamic web app only using a frontend framework and js. I mean how are they even handling their backend? I am really curious to know this.