The only thing that matter is whether you trust the app or not.

- If it is proprietary, you just have to blindly trust it (as is the case with WhatsApp currently: they say it is end-to-end encrypted, but you can't verify).

- If it is open source, then some people will want to understand how it works before they trust it. Other will either blindly trust (like for proprietary software) or trust that persons they trust understood how it works and were convinced.

> You might as well ask, how does $browser work with HTTPS?

Well, exactly. I am interested in how the WhatsApp interop works just as I am interested in how HTTPS works.