He basically admits that he’s been picking over both battlegrounds and boneyards to create these 1:1 replicas. He said that the saucer had to be recovered because of The Prime Directive, apparently there was a native pre-warp species on the planet they crashed the saucer section onto, and the nacelles, pylons, and warp core came from the wreck of the NCC Saratoga (?). I suspect that would be the entire Battle Section of The Saratoga, since the Battle Section of The Enterprise was thoroughly destroyed, so that includes the main deflector dish, main engineering, the engine room, and something like 7 decks of the ship.