Not all the tech. Only very specialised one's like Teleportation.
Ships and weapons are made by templates and what no set before hand and most of it is known to them or known from reverse engineering. Again, certain specialised weapons and vehicles cannot be recreated and thus are only used when of vital importance.
Oh. It's such a cool universe, god damn. Bleak as shit. Who's the space marine Jesus looking guy in the gold armour with the flaming sword then? I always thought he was the emperor, not the rotting corpse wired to technology.