Is the OS that big that it will use up 12-13GB of the 16GB internal storage?
One thing to remember is that DiskUsage shows storage in binary units. However storage vendors and many apps, including probably your system menu use decimal units. This adds an extra layer of confusion to such discussions (DiskUsage labels its units "GiB" to try to indicate that it's using binary, though really "GB" is perfectly correct for binary units. I'll adopt this convention here as well for clarity, though I dislike it myself).
The difference is that a binary G is 2^30, while in decimal G means 10^9. So your 16 GB storage (16*10^9 bytes) is actually 14.9 GiB. So if DiskUsage is showing 10.8GiB total that means that 4.1 GiB is being used by the system partitions. That's 4.4 GB in decimal units (relatively small for a Samsung).
What it's showing as "system data" is harder to say, because without root you can't tell what it is. It may be actual system data, updates to system apps (system apps live in the /system partition, but their updates eat into the user-available space), for all I know it could even be data related to user-installed apps that DiskUsage doesn't have permission to look at. 8.5 GB, if that does not include the "missing" space (system partitions), is however rather large: on my ancient Galaxy Note 10.1 tablet (my only Samsung) "System Data" in DiskUsage is 1.2 GiB, while on my 3 year old and heavily used Pixel 2 it's 1.6 GiB. So it's that 8.5 GB, rather than the "missing 6 GB" (really 4.1 GiB/4.4 GB) that's surprising to me (though if you've included the "missing" space in that 8.5 it's not so crazy).
To add one further confusion, other apps, including your system storage menu, may label things differently (both in how they define "system storage" and what units they use/how they label them). This can make it very hard to reconcile what different apps and system menus show.
