Technically it’s both. It’s just that the bones stand out much more than the soft squishy insides.

All of them are XYZ of the head,

There are brain x-rays called pneumoencephalographs where they replace the cerebrospinal fluid with air, x-ray, and then put the fluid back. Pneumoencephalography and angiography were really the only neuroimaging techniques available for most of the 20th century, so it was pretty common.

I asked mydelf the same. I guess you can only x-ray the brain by itself when it’s not enclosed in the skull? So this should be the best resolution when scanning a living human.