WhatsWeb working procedure | Technology and so on?

Can someone please explain how the web.whatsapp platform is works.

As if i disconnected my mobile from internet then it will not work.

What technology they are used to implement such kind of scenario. And also how they clear the notification after i read a mesaage from web.whatsapp or vice versa.(does they use web sockets?)

And does they used my mobile data? If yes then what happened if i have slow network or i am behind proxy.

As I want to explore and try to implement in language like node.js or python.