How to access system files which has data of all the installed apps(eg: videos, pics sent/received, whatsapp data, delete un installed apps backup)?
Use Files app from Microsoft available in AppStore, which allows to browse your phone and SdCard memory where you can delete Whatsapp backups which is stored either in your Phone or SD Card as your suggested to keep backups while installing apps in a folder named by the apps (e.g. : Whatsapp backups will avaiable under Whatsapp folder, so on)
The easiest way is to use a Card Reader for the OS and app data on your SD card. For your phone memory you will have to get developer access (you have to pay for it I guess).
For developer unlocking go: