i would imagine that on a production line it will be a standard 2k primer the body maybe cleaned with acid, neautralised, washed and dried prior to priming.
as for bodyshops they will use a variety acid etch, 2k and epoxy they may well prefer to stay away from acid etch because of residue acid that could make the subsequent coats fail, they may stay away from epoxy aswell due to time restrictions i suppose its down to choice and time they want to spend on each job
When I ran my bodyshop, production repairs to Thatcham times, ally would get wet on wet etch/high build 2k, guide, 800 top coat, out. Steel panels I would miss out the etch. There was a world of difference between Thatcham times and doing it properly.
Nick