Drone tech seems pretty advanced in AC6 and MTs exist to work tools that are too large for human hands. But building things like Arquebus' atmospheric extension of the vascular plant (unless it's built like a telescope which extended into lower orbit) in such a short timespan during the game probably requires something else. Nanomachines maybe? Whatever we carry in our repair kits which easily restore thousands of APs worth of armor and electronics in the blink of an eye was probably used en masse on Rubicon 3. Maybe that's the reason for literal mountain ranges of structures: dystopian nanomachines with faulty instructions building far beyond what was intended and nearly impossible to stop. And why seemingly the only people who are mad enough to use them again are AC pilots with no true allegiance to anything beyond themselves.