I do not agree that these technologies are not understood. They often relied on a generous application of a labor tax to make things happen. For instance, that three thousand mile highway was surely built by the communities along it up to the imperial standard. Thus in any given year tens of thousands would work on the road for a set period. If your village was responsible for a twenty mile strip, a month's work by fifty men will groom all the easy parts quickly. After that it is bank building with stones which is much slower but not so much so either. Then we have step building and the like and that can be handled best with a handful of men every year. My point is that it is all feasible and a rough viable road was open early.
The fact that you return to the task every year as well encourages a steady improvement and continuing damage abatement.
Stone cutting and stone surface matching precession remains partially explained but then they had ample time to develop skills to perfection. We cannot discount something so easy to plausibly perfect. We are also not describing a massive number of rocks either.
That these accomplishments can be emulated is obvious although manpower will not be so readily marshaled.
Ten Things the Ancients Did Better than Us