My iPhone 13 pro max automatically turn on wifi

How are you turning off Wi-Fi? If you are using the button in the Control Center that does not turn off Wi-Fi at all; it disconnects from the current Wi-Fi network, but leaves Wi-Fi on. To turn off Wi-Fi go to Settings/Wi-Fi and turn if off there.

As far as WhatsApp is concerned, that is not an Apple product. Go to the WhatsApp support page→https://whatsapp.com/faq

I’d like to add that simply asking Siri; "Turn off Wi-Fi" also works. 

Good point, thanks.