How to open an external app from react native app?

You have to use React Native Linking API:

Linking.openURL('whatsapp://app')

Also, have a look at How can I integrate WhatsApp into my app

Previously below was used,

Linking.openURL('whatsapp://app');

But later the URL is changed as follows, we can also add phone and text query params shown below,

Linking.openURL(`whatsapp://send?phone=${whatsappNo}&text=${whatsappMsg}`);