I'm beginner programmer in general. Is it possible to add files to Android app folders in /data/data using my app on a non-rooted device? I'm planning to make an app using C++ and I’m not using Android Studio IDE, my app should add whatsapp stickers to whatsapp.
No, for security reasons. You can put files in your app's portion of internal storage, but you cannot put files into other apps' portions of internal storage.