How to create System Apps in android
True system apps are baked into the ROM image, and signed with the same key the ROM was signed with, giving them extra access to the system.
You cannot add a new system app on the same level without updating the ROM image of the device, and even then you'll need the OEM key to sign your app and you will have to request them to include it in their ROM.
Having a rooted device allows standard apps to gain extra access without being system apps.