How to completely delete an app and its data?

I noticed that deleting and reinstalling an app brings back all its data as if it was never deleted.

I tried all these:

• Delete from the home screen, tap and hold, tap x
• Delete from settings > iPhone storage > delete app
• Turned off and deleted iCloud backup
• Turned off iCloud drive
• Hard reset the phone after deleting the app

Nothing seems to work. The second I reinstall the app, all data comes back, it does not even ask me to log in again.

To test I delete Instagram and Facebook using the methods above, and they behave the same. As soon as I reinstall all data is still there.