If I use WordPress REST API V2 and someone makes an app using it. Will my site count the posts views from the APP? And if not, then how?

WordPress does not count views or traffic, it’s not a part of WordPress’ feature set. Plugins and code snippets can add this but it’s highly specific to the service you’re using.

Usually for apps you will need to integrate tracking into the app itself. But how you would do that is specific to the service you choose, not WordPress.

To answer this you will need to consult the documentation for the service you’ve chosen to integrate. There isn’t a WordPress based answer.