WhatsApp is end-to-end encrypted. WhatsApp actually uses the Signal Protocol. Probably the best protocol they could use. It's described pretty detailed in a whitepaper publicly available on the WhatsApp website: https://www.whatsapp.com/security/WhatsApp-Security-Whitepap...
Ofcourse WhatsApp is not open source, as opposed to Signal (the app). You can never be truly sure if the protocol is implemented without backdoors because you can't verify the code. But I agree, WhatsApp is a pretty solid app.