[BwanaNdege]

Do any of you know what happens to the human head during a cremation?

Think coconut.

https://copicmarkertutorials.com/5-weird-facts-about-the-human-skull/

2. The human skull is full of “holes”

Besides your peeper holes, your smellin’ hole, and your pie hole, the skull is full of tiny holes called foramina. These openings allow for the passage of the nerves and blood vessels that innervate the face. The largest hole in the skull is where the vertebral column joins the base of the skull, and is called the foramen magnum.

Lots of holes mean no pressure build up, i.e., no "coconut explosion".

[Slyfox]

That is not what a crematory technician told me. I won't describe exactly what he said but let me assure you that when the temperature is high enough anything that looks like a human head will bust open quickly.

Besides, the holes you mention have a number of tissues between the inside of the skull and the outside of the head that impede a direct connection to the outside world.

If there were no such tissues our brains would get easily infected.