I'm currently reading Gateways To Art: Understanding the Visual Arts and in the introduction it mentions how there was a time that Renaissance artists staffed workshops of artist assistants who “carried out most of the work involved in turning the master's design into a work of art.” It doesn't go into further detail on the subject.
How involved were these artist assistants? Did they help paint the actual works? Or were they there to prepare the work for display? To what extent were they usually involved?