Can a app store data beyond uninstallation? I've an app that creates an account for me but uses no login credentials (starting the app logs me into the account). 


When I uninstall and reinstall the app it still logs mit into my account.
When I reset my Android Phone (e.g. new OS) it does not remember my account


My question is: What possibilities could this app have to do such behaviour and how do I eradicate it completely from my phone when uninstalling it?
A lot of apps leave behind what I like to call junk files.

There's an app called SD Maid which helps you clean up your F.S. (filesystem). There is also Ccleaner and System app remover, these apps will assist you in cleaning up cache and removing apps. You can also navigate to /sdcard/Android/data/ and remove any left over folders an app may have left behind.

What I like to do is clear the apps cache and then uninstall the app. I hope this helps.