Well to answer your question, I think it is important to first read the insufficient-memory tag wiki to get some insight.
A significant amount of memory in internal storage is taken by "system" (
/data partitions etc) and in some cases is reserved memory.
Some manufacturers include preloaded applications (sometimes known as bloatware) and these reside in
/cust (in my case) partitions. It is virtually impossible to view the breakdown of these files without root privileges. Some space is reserved for ROM updates, acts as system buffer or caches storage etc.
While it's difficult to uninstall unnecessary applications residing in these "system" partitions, there are a few methods that can help you optimize your space
check for pre-installed apps you don't need. Clear their caches and
data, then uninstall their updates and finally disable them (so they
won't get updated again).
While pre-installed apps reside in the /system partition (which you cannot utilize without root), their data and updates consume space on the /data partition which gets freed this way.
If you have acquired root privileges try moving some apps to other storage (SDcard) via App2SD