If you are looking to build your own WhatsApp like the app then I will suggest you one amazing software that can help you in building your own WhatsApp.
WhatsClone Messenger is an Android app that supports almost all mobile phones. It is build using NodeJs & Mongodb, for client-side App is using Room as database local.
Using WhatsClone users can do is a real-time chat with friends (Like WhatsApp). You can send and share (images or videos or audio or pdf) etc. there is a database local to save your messages, so when user lost the internet connection he can write a message, the message will be saved on database local and once the other user comes online the app will send your message directly.
Also, you can delete conversations or messages. Feel free and enjoy the new emoticons. For the Backend side, you can control your app as you want even you can control and settings up the Admob from the dashboard.
If you are interested then you can buy this application from here
Secure Chat app
The secure chatting app is also a WhatsApp clone but has some different feature, App uses your mobile internet connection to send messages to family, friends, near and dear ones.
This is a full Real-time application that comes with humongous features benefiting every user in every way they need with secure and fast messaging protocols.
React Native Chat App
This is a react native app. Full native (android ios) real-time chat app. It is very easy to use and app is available for both Android and Ios users.
If you are interested then you can buy this app from here