Company A wants to hire employees to perform Task A. Task A has a lot of legal overhead or paying taxes for employees and Company A would rather not deal with that. So Company A hires Company B as a contractor, meaning Company B is primarily responsible for the legal overhead and tax liability of Task A and Company B's employees. As a contractor relationship, Company A is not able to demand Company B do certain things (such as Company A dictating what Company B pays employees, or setting PTO policies, etc). Usually, they are only able to set standards that Company B must meet to maintain that contract for Task A.
It's possible Company A here (Google) may have overstepped and began instituting requirements or policies on the employees of Company B, which leads to the misclassification of employment.
Google may say "They're not our employees" but if they're overstepping their bounds then they might actually be considered Google employees. If it walks like a duck and quacks like a duck, I can say it's a pigeon all day long but everyone knows it's a duck.