https://www.cnet.com/roadshow/news/how-gm-cruise-automation-trains-its-drivers/

Its drivers, which GM refers to as Autonomous Vehicle Testers (AVTs), are responsible for ensuring the safe operation of its autonomous fleet of Chevrolet Bolt EVs. More specifically, they have to engage the self-driving system, record behavior observations, provide feedback and take control when necessary.

The Bolt EVs used for autonomous development are the same ones that roll off the line in Lake Orion and head to dealerships, albeit with a few extra pieces of kit.