I believe Crooks went in days early with the long parts of the gun and hid it somewhere around the air conditioner unit he climbed. Possibly it was already assembled except for the scope. He used the drone before the rally to make sure it was still there undisturbed. Once he got to the air conditioner, he put the gun in his backpack so he could climb easily, reached the roof, threw down the backpack and attached the scope to the gun.
1. Whoever was in charge of inspecting the area in advance either blew it, or was in on it.
2. There must have been a plan B in the event the gun was discovered and removed.
I was a reflex sight, no magnification. If he did do it this way, he'd be lucky to have MOC (Minute of Crowd) accuracy. If the cluster of shots around Trump when he was hit were actually fired by tranny boi, he didn't take the sight off after practicing.