During deployment on an Android device, the Android system will create a unique user and group ID for every Android application. Each application file is private to this generated user, e.g. other applications cannot access these files.
Some permission is automatically granted by the Android system, some are automatically rejected. If the user denies a permission required by the application, this application cannot be installed. The check of the permission is only performed during installation; permissions cannot be denied or granted after the installation.