so, I'm really new to bender and I'm trying to make a ring for myself. but for this I have to remove part of the skull, but when I try to use the boolean difference modifier it simply doesn't change anything. But it doesn't act like it didn't work.
The topology of your skull is bad, there are vertices overlaying and inner faces, f you select some external vertices in Edit mode then press H to hide them and see what happens inside you will see it by yourself. Boolean doesn't work with overlaying vertices and inner faces, and I don't know if there's a quick way to fix the problem as there are a lot of vertices.