Help Android Storage is full, but no files

[JoeyJohnJo]

So, I have a 32GB Samsung Galaxy J7 model J700H. I have used only 2GB of space including all my apps and misc files, but, in the storage tab, in Settings, it shows I only have 400mb, of space left when I should have 30GB left. Furthermore, if I try to install an app from Google Play Store, it warns me it is not possible due to the lack of space. But if I install the same app through an APK file, it works.
In the Storage Analyzer app, it says that 30GB is used by Android, but the ROM itself can't possibly be that big. How can I free up this space?

 

[Hadron]

The ROM isn't that big. Moreover, the ROM doesn't get bigger: it has to fit within a partition that's a fixed size.

So, the Storage Analyzer app isn't being helpful. An app called Storage Truth (which is tiny) will show you the reality: the space you can use is the /data partition, which is a lot smaller than 32GB (because up to 10GB of that will be used by the /system and /cache partitions plus the others that hold lower-level firmware - that doesn't mean the system is that size, there will be free space in the partitions to allow for upgrading, but you can't use space in other partitions). As for what's using space, what does the system Settings -> Storage tell you?