Some phones will not allow the movement or download of apps to the Sd card @Jumanne which is probably the case for your phone model.
They will go to internal storage.
If your downloading apk files are you sure your downloading from the official Play store.
With the internal storage only being 4Gb with also some of that 4Gb being used for the Operating System the phone won't hold alot so moving music , pictures etc will help free up space along with clearing cache and data and removing emails and text messages you don't need to keep.
Current Phone >
Samsung Galaxy s²³ Ultra 512gb Phantom Black.
