How do I delete iOS storage on my 6 plus

It is not possible to clean up System Data or iOS Storage yourself.

System Data is cache and system files from iOS and the apps you have installed. These files help launch your apps faster and ensure you get the information you want without waiting. Over time, the size of System Data will both increase and decrease. iOS automatically clears unnecessary files to make room for new ones.

Go to Settings → General → iPhone Storage to see what's taking up space. Remember there is a difference between the space on your iPhone and the storage space in iCloud.

The iOS system takes up 7 GB, then the system files come after that.

So there is not much space left when the iPhone only has 16GB of memory. It doesn't look wrong, but unfortunately the system files just take up more space today than they did 7-8 years ago. All you can do is delete what you don't use and keep what is absolutely necessary. Alternatively, consider whether it's time for a new iPhone.

Unfortunately 16GBs is not much these days. FWIW, I am on iOS 16.31 and I have 9.46 GBs for iOS and 5.72 GBs for System Data so your numbers do not seem abnormal. I have plenty of storage as my phone is 256GBs.

I have not tried this recently but in the past one could reduce the amount of Other (now called System Data) by backing up the phone to a computer, Mac or PC, and then restoring that backup to the phone. That takes time and I am not sure if it still helps. In the past it would reduce the System Data but only to start growing again slowly.

The only other thing I could suggest is deleting some of the apps you rarely use and then doing a Forced Restart.

How much total storage do you have? That may be normal as iOS self manages the storage.