Hello Whatspoppington,
It sounds like you have some concerns in regards to your cellular data usage.
To "find out how much cellular data that you're using, and learn how to turn cellular data on or off for all apps or individual apps" you can check the cellular data usage on your iPhone and iPad.
Based on the information you provided we can offer the following recommendation:
To "View how much data you're using" go to Settings > Cellular or Settings > Mobile Data.
You can "scroll down to find which apps are using cellular data. If you don't want an app to use cellular data, you can turn it off for that app. When cellular data is off, apps will only use Wi-Fi for data."
We recommend toggling the STARZ app off if you do not want it to use your cellular data.
We also recommend making sure you have Wi-Fi enable and connected. To learn how to ensure your device is configured properly to connect to a Wi-Fi network, please refer to: Connect to Wi-Fi on your iPhone, iPad, or iPod touch
We appreciate you reaching out to Apple Support Communities.
Be well!