check this website there are a bunch similar to it, that has already prebuilt templates, and its ready to use basically, android has one also but for now i think this is what you should looking at.
https://www.appsgeyser.com/ they even have a whatsapp clone template there.
BTW people above are right, you might better need to do it by hand, i wold do it in Ruby on Rails, Twitter/Instagram, i think even whats-app is built in it. much faster, and can handle tons of users.