How Can Medical Examinations Be Done On People Who Have Been Dead For Decades or Even Centuries?

In the case of hypertension, they would probably need most of the tissue to be intact. That would be likely if they replaced the blood with embalming fluid which used to be something like formaldehyde. The organs were probably still intact. You don't die from hypertension itself. You die from what it causes like an intracranial or brain hematoma which is a burst blood vessel in the brain. They could have found signs of that as a large area of coagulated blood inside the brain. Many diseases leave marks or impressions on the bones and that is how they can detect them. Some like certain poisons will still be in the body or detectable in the bones. When they find damaged bones, they can tell if they healed or there was an infection which scars the bone. If the bone healed then that obviously didn't kill him.