Data/data can be manipulated. Under this folder are SMS, call records and qq data.
The system /app is a system application, just don't delete it casually.
System/Font This is a font file, and beautifying fonts is operated in this folder.
System/lib driver folder, please make sure to delete only files you know.
If there is a system/preloaded folder, delete it directly.
You can delete the system/media directly.
You can expand your system, take a screenshot, let me have a look.