How the Megalithic walls of Cusco were made - using pendulums

The answer is: large rocks were suspended in the air and ground together systematically, one rock after another and/or several rocks at the same time. Here, this picture proves it because it shows exactly the sort of partial grinding that must have occurred from time to time:

Foerster does not spend much time, talking about the obvious finishing effort that smoothed away the handles used for attaching the rocks and, further, finished the grooves off to eliminate ragged edges between them. The finishing of the grooves is evidence of post-placement finishing. Once that is admitted, then removing the handles becomes a possibility.
Here is one of the handles that would be easy to remove afterwards.

I write the comment: After watching your videos it becomes obvious that a VERY extensive finishing effort occurred after the stones were placed. The grooves were finished afterwards, so the knobs might have been removed as well. Thus, suspension from ropes becomes the obvious method for grinding large rocks against one another with little or no effort using pendulum motions.