I just found this video, and likely this artist learned through apprenticeship, and techniques may have been similar to ancient Greece/Rome. (An interesting question to ponder: was it an unbroken lineage of craftsmen or was the skill lost but then rediscovered during the Renaissance).
This is a must see video for any art lovers or artists:
Master sculptor Fred X. Brownstein recreates Antonio Canova’s sculpture “Venus” using Canova’s unique 200-year-old techniques over the course of 6 months.