There are two types of stroke. Ischaemic where you have a clot and it blocks an artery/blood supply to the brain and parts get damaged. Haemorrhagic where a blood vessel or aneurysm bursts and fills the space with blood causing damage.

It’s likely he had the later, because sometimes to fix it they open the skull, remove the blood, release pressure and/or clip the bleeding vessel to stop it. Often the surgery is done in a hurry and the skull is left open (with the skin sewn back on, in case they have to rush back to fix it. Leaving a dint in the head.

Now it’s stable they’re replacing the piece they took with an implant, usually metal but some newer ones are 3D printed like his was.