What, exactly, is a doornail, and why is it dead?
Most nails are straight with a sharp point and you can lever them out using a claw hammer or other tools. However, a doornail from the period where all doors were solid wood nailed together as the only way to secure together were much longer than needed secure the boards together. The top of the nail was usually rounded so it was hard to work on once in place and it was long enough to give an inch of excess nail on the other side after being hammered through all of the wood in the door, then the excess was hammered down flat to the wood on the inside. Thus with the round top hammered to below the surface of the outside wood you couldn't get a chisel under it easily and the long tail being hammered down at more than a right angle to the main shaft so the sharp point was now set under the surface of the inside wood meant there was no way you could move the nail at all without seriously damaging the door and making it useless.
Also, as stated in:
https://grammarist.com/idiom/dead-as-a-doornail/
quote
Dead as a doornail is a phrase which means not alive, unequivocally deceased. The term goes back to the 1300s, the phrase dead as a doornail is found in poems of the time. ... It is thought that the phrase dead as a doornail comes from the manner of securing doornails that were hammered into a door by clenching them.
end quote
On a personal level I've often wondered if it was just a variation of the other saying 'Dead as a doorpost' as a doorpost was one of the most securely set pieces of wood in the old buildings and it had to be so well secured it didn't move when the door was slammed shut and it had to stay secured when someone was trying to break the door in. Thus the door posts were usually much longer than the door and were extremely well set into the floor and the wall around the door to give the maximum strength against an attempt to break in, so it was not moving at all and seen as totally dead. The walls had some movement and were seen as alive while the doorposts had no movement and seen as dead. Most wood buildings from olden times had some movement in them in the walls, but not in the doorposts which is why the doorways were usually the last to go as the building deteriorated.
To add a little trivia to what Ernest posted. According to my source for all things carpentry, when nails are driven through the wood as described above, the apprentice would be instructed to 'knock 'em dead', meaning to hammer them over sideways and as Ernest stated, ensure the point of the nail is well embedded, partly for strength, partly to avoid a snag as someone brushed against the door. I'm told that if the apprentice drove the nail across the grain instead of along it, he would have 'killed' it. Apparently because doors were constructed from hardwood it was much harder to get a nail to properly 'bed in' when across the grain, which left a weaker bond and one much more prone to be a 'snag'.
More trivia. Nails of that period were individually forged not mass produced. Obviously not a task needing a master smith, they were often made by the apprentices, who would form the heads in a certain way so as to be able to identify them as 'theirs'. The heads were, as Ernest stated, often rounded but were also made square, and with three or four raises sides rather than a simple dome. The shaft of the nail was usually square in section though triangular shafts were not unusual.
Some more trivia.
The first recognized archeological example of a nail was found in Egypt. It radiodated to 5,400 years ago.
There was another older(?) example but it was not recognized by many. It was a copper nail reported to have been found in the Kashmir region of India. As I understand it, there were insufficient radionuclide decay products to positively identify how old it was.
Either way, the common nail predated recorded history.