I have a Samsung Galaxy J5 Prime. My internal storage is 16Gb, it is almost always at either 15.8, 15.9 or 16/16. I clear cache on a daily basis just so I can do things like go into one of my games or take pictures. My applications altogether take up 3Gb, I move images to my SD card so they only take up 20mb internal at the moment. Videos take up 0, audio takes up 8mb.
What takes up the other 12Gb on my phone? It can't all be RAM, can it?
The phone's "Internal" storage doesn't actually have the full space that the phone is advertised to have. The space listed is before you add in the OS, all the preinstalled apps, etc.
So you actually only have access to a small portion of that 16GB.
Personally I think companies should have to tell you not only the full amount of storage, but also the 'available' for use amount. Rather than just telling you the total and not mentioning that a lot of is taken up before you even get the device.