The door proper, the prisoners can’t touch. And as one can see from the news pics is a normal door.

The bolts in question are the assembly and mounting bolts for the “cage”.

How does mm one hit their head on one?

Intentionally does it, banging and thrashing around.
Unintentionaly when the vehicle starts or stops fast.
The prisoner isn’t seat belted in and is allowed to move around the cage eben semi freely and falls against the bolt..

The prosecution is throwing the charges heavy as its almost impossible to add charges but easy to drop them.

Much will depend on the legality of the original arrest, if it flies, the worst they may get is criminally negligent homicide, for not belting him in.

Again, until all the facts are in, its difficult to get the full picture and sequence of events.