Like field stone fences, they found or created blocks of varying sizes that could be fitted. They then cut/chipped the stone down to a pretty close fit and applied a thin, special paste mortar when the stones were set. When wet the paste dissolved the last few millimeters of the rough faces of the stone, the paste washed out and the stone fit together with microscopic tolerances.
The close fit of irregular stone made the walls very earthquake resistant. Really freaking clever.
Is that some archeological speculation (as thinking “it simply had to be that way”) or somehow known that that is what the Inca did (with stone)????