Other and System storage-after restore is still a lot??

So, I’ve been trying to free up space in an old 6s plus I own that I have as a backup phone and the capacity is very chinsy at 16gb as it is. Even after signing out, removing all iCloud data and resetting to factory settings, upon activating the phone afterward with no user or iCloud linked or SIM card inside, just a generic basic bare bones setup process, I checked in the iPhone storage afterward and it showed that 8GB of 16GB was all that was available for use and of that used portion was all but less than 1gb used by the preinatalled apps with 0 data associated with each system pre-installed app (messages, photos, camera, phone, safari, etc.) basically the apps themselves using that space, the rest was all “system” and “other”. Over 7GB is being taken up by system and other and none of the system or other include the apps that come preinstalled on the phone as mentioned above. Those apps are listed and the amount next to it that is using storage. This also is with Auto updates turned off both download and install with the latest 14.7.1 iOS installed previously with no updates saved on the system storage waiting to install, which I do know ends up being what most instances of this problem are attributed to and installing the downloaded iOS update will free up that space. However in my case, no iOS updates needing installed or had been downloaded, latest iOS installed, general reset with no user attached or signed into phone, and zero data history because I had just started after completely resetting and removing all previous data and started fresh. Still, over half of the storage capacity shows used by system and other. So I then try to do a textbook common restore through iTunes process.

I ran the phone through the standard restore and update via iTunes process and successfully completed that and once again setup generic with no iCloud or user and no SIM card inserted and signed into WiFi. Disconnected from my computer and after reaching the Home Screen and checking in settings to see if the build up storage in other and system had been dumped and freed up much needed space and would allow more than 8 gb of available space when a user signed in and possibly use the phone without worrying about storage, lone behold there in the used space was still only 8 gb left and the majority of the 8 used was from other and system but I just don’t understand why or how because there was no additional data being saved or any user data at all and the iOS still running latest version, auto updates off both download and install, and nothing yet had been used or installed after restoration. I pressed the power and home button keys simultaneously for the amount of time it took to prompt DFU/recovery mode while plugged into a computer running latest MacOS and downloaded and installed latest iOS to current and latest version iTunes application and a successful restore was performed. This should’ve wiped out that system and other bull crap but for some reason, is still there. Please, someone, enlighten me on this subject as it is a very common and confusing question I am asked daily by my clients-by the way, I also am an independent consultant for mobile device and most computers technical support agent for Apple, Android, and Microsoft products and have not yet been able to figure this one out. Is there even a way to get rid of this built up system cache and other wasted and unnecessary space being taken up on the on-board iPhone storage ? Or does the phone just carry over bits of data from the previous user since the first time setup and store it in this area inaccessible to the owner and unable to dispose of no matter what? Basically in theory, the older and more used a phone is, the higher amount of storage used in other and system shows even after previous users and owners have no longer been associated with the device there still might be bits of data lingering since the phone was first ever used or setup? I can’t think of any other explanation as I’ve seen this same problem again and again and it seems like the older a phone is, and heavier it was used, tends to have higher amounts in that system and other storage areas even after restore and factory resetting for a new user to sign in. This is especially frustrating with the lower storage capacity older iPhones and is very strange nevertheless in comparison to other phones undergoing same process.

Any suggestions how to fully restore and remove the other and system so that I can fully make use of the measly remaining gb of storage I have after preinstalled system apps and data have been accounted for which I’m estimating should at least be about 12-13gb of 16gb available for use… thanks for any and all insights!

Angel