How to create an API for someone else's application?

Question

Mobile development

Hello! Asked such questions, he does not give me rest.
Can you help me understand if this problem can be solved or not?
In ios and the play market, you can often find copies of instant messengers, but with their own additions. For example, whats up for a tablet or some modified viber. Which use the api of these messengers.
And if the messenger does not have an API, is it possible to get its inner kitchen? That is, where does he send what request and receive it back in order to make his own kind of api to it and build his copy of the application on this basis?
Thank you!