What determines whether Google is really a 'co-employer' with the subcontractor?
one example: workers in this bargaining unit working on Youtube Music at Cognizant as contractors for Google have to request vacation first from Google, and only once they agree, can request their vacation from Cognizant.
A complete formal definition of joint employers is in "Decision and Direction of Election" from https://www.nlrb.gov/case/16-RC-305751 on page 8. The document also has more summaries of evidence and examples from the hearings, which were non-public.