Indicators

To extend deggie’s answer...

So-called immersive apps can mask the status bar, and there was a bug back around iOS 10 that also allowed that to happen.

The app developers have control of the visibility of the status bar:

https://developer.apple.com/design/human-interface-guidelines/ios/bars/status-bars/

You need to contact the developers of all the applications that are doing this and ask them to include these indicators in their apps. Are you having trouble remembering who your mobile operator is?

Update your device to the latest version. Every thing will be fine.