Trump seems to think that turning an existing production 747-800 into an AF1 plane is simply a matter of changing the seating layout and the paint scheme.
The AF1 747s are built with vastly different communications and avionics so they can serve as secure flying command posts during emergencies. They can be refueled in mid-air (a tanker plane follows it around when the president is on board, just in case).
They're also used in pairs, with the other plane on standby at a nearby airport.