Mobile storage space 4 GB too low?

There are many questions about disk space issues, but I have not found any answers to my problem.

I have a Huawei P8Lite 2017 with 16 GB of internal memory. In addition a 32 GB SD card. The latter is a standard memory for pictures, videos, etc.

So the internal memory is only for apps and system files. But now for two weeks I have the problem that "too little space is available.

I've deleted apps, emptied caches, etc. And yet my phone seems to be trashing itself. Under the storage management I looked at the shares of the storage distribution. The fact that about 7 GB alone go for firmware apps, of which I do not even use a third, but can't uninstall, I think so outrageous enough. But if I compile all instances I come to a total of 12 GB (mind you including the free space of only 500 MB!) And I know that for all storage media of the specified storage space (8, 16, 32 GB, etc) never to one hundred percent exists… But 4GB of 16 ?! That's a quarter, that can't be!

Who can help me?

I do not know if that helps in your case:

On one of our cell phones, the e-mail program stored all mails with the supplements hidden somewhere in Hautspeicher. As a result, the memory became full quickly, "out of memory".

I do not remember exactly, it was already a year or two ago. I've found the files of the mail after a long search and rausgeschmissen. I found the app, disabled it. It had not been possible to delete it:-(. Have another mail app in, since then, rest is in Suastall.

There are also other apps whose data Android does not recognize as data, so it does not list. Something with the endings of the files, the ".xyz".
For example, this is the case with the osmand app. Map and Navi app. The data of the maps are from openstreetmap.org.
When you recharge your cards for the whole of Europe, the memory is very quickly full, and you do not know why… The Harre breaks down.
My Honor 9 has 64 Giga internally, a good half is still free, and has 64 Giga card, only about 35% occupied, there it goes:-).