How does the web version of Whatsapp work on iOS devices considering the OS shuts apps in 30 seconds?

Question

Now for those who don\'t know, can go to https://web.whatsapp.com/ and sync your Whatsapp chats by exchanging a QR code and chat via the web extension of the ap

Answer 1

As per the docs the app can remain in the background performing finite updates to the App. You can continue extending the background process one after the other. Look into Perform finite-length tasks. I think killing the app from background still executes registered actions by the system.

Whatsapp does some clever web session token + background app token generation to keep session valid.