How do i delete ALL the app data even after uninstalling the app?

There after a few apps which seem to find a way to keep some identification data or something like a cookie on the iPhone even after deleting the app from the iphone. How do I know? I uninstall / delete the app completely from my iPhone. So in theory all data related to the app should be removed from iPhone (I’m not talking about data from the app servers, I’m taking about data the app has stored on my iPhone).

So now when I re-download the app again from the App Store it should be just like a brand new install, ie the app should not know who I am or anything about my previous interaction just after a fresh download, it should present me with a page to create a login or sign into my account. But I’ve noticed that with some apps the moment I open the app after a fresh install it seems to know exactly who I am and prompts me to use my previous profile to log into the app? If all the app data is deleted from the iPhone how does the app know who I am or anything about my previous usage just after a fresh download? I’m where is the app storing this temp / cookie information that persists even after an app is uninstalled?