You can control your own mobile device settings. It is important you learn to do this so that you can manage data usage and security. You can:
Your mobile phone operating system also needs internet access if it is to receive updates to the operating system. You can choose to download updates automatically or manually. Operating system updates are large consumers of data. Ensure your settings do not allow this over mobile data. If updating over Wi-Fi this may be free or lower cost than over mobile data.