The exact details are classified, but, if the current aircraft are anything to go by:
Military-grade sensors, telecommunications equipment (the current aircraft have 87 telephones and 19 televisions each), additional exit points, a completely remade interior, including the President's private quarters, a kitchen, storage equipment, cold storage equipment to store enough food to serve hundreds of people (or a few dozen people for several days, there was an article about a $20M fridge a few years back), anti-nuclear shielding, reinforced glass, separate quarters for guests, senior staff, Secret Service and security personnel, a medical annex, which includes a fold-out operating table, emergency medical supplies, and a well-stocked pharmacy, the president's executive suite, a conference room, modified engines.
Air Force One does not have a fighter escort most of the time.
I would describe it more as made-to-order / custom rather than over-engineered. Over-engineered would suggest that the complexity of the engineering is unwarranted by the requirements. I think in this case, the issue is just that the requirements are very high, and since the requirements come from the customer, they simply must be met.
If you consider the military the end customer, sure. But take a step back and the military is the just the defense vendor for the People. If the People don't need their president to be supplied with all that tech, then the military has over-engineered their solution to the People's "our president should be able to move places relatively quickly and relatively safely" problem.
The President needs to be able to do their job as President from these planes while in the air, including some of the more unique responsibilities of the job like directing a nuclear war. The requirements go pretty far beyond just providing speedy and safe transit.
My point is that the presence of unusual or even absurd requirements is a different thing than over engineering, even though the results can seem similar. If a design incorporates complexity that is not warranted by the requirements or the constraints, it can be described as over engineered. Requirements can cause the same complexity, but it is now warranted.
You may, of course, disagree about the requirements, but that really is a matter of risk tolerance, and preexisting policy.
I've wondered about that. Protecting the president's life isn't worth infinite investment, but I think the planes also serve as command posts during warfare, including nuclear war. That's worth quite a bit.
Military-grade sensors, telecommunications equipment (the current aircraft have 87 telephones and 19 televisions each), additional exit points, a completely remade interior, including the President's private quarters, a kitchen, storage equipment, cold storage equipment to store enough food to serve hundreds of people (or a few dozen people for several days, there was an article about a $20M fridge a few years back), anti-nuclear shielding, reinforced glass, separate quarters for guests, senior staff, Secret Service and security personnel, a medical annex, which includes a fold-out operating table, emergency medical supplies, and a well-stocked pharmacy, the president's executive suite, a conference room, modified engines.
Air Force One does not have a fighter escort most of the time.