One is the iOS itself. The other is the supporting files like caches and temp files/logs etc that are created/deleted depending on the need by the OS. Consider on-device machine learning for example.
You can’t delete either of them as they’re both inaccessible by user and are required for your iPhone to run. OS’ are a lot more complicated than they used to be and are likely to get bigger depending on the addition of new features.
Why are you so bothered by that if you don’t mind me asking?