My insurance has specifically told me that they will cover me if I am driving someone else's vehicle. Now they may go after that person's insurance for reimbursement, but regardless, I am covered. I know some folks whose insurance will specifically NOT cover them in any other vehicle but the one on their policy.
It's all very convoluted and there is no "one way" regardless of location, company, individual. (For example, within the same company and same location: a sibling was not covered driving any vehicle but the one on her policy; I was covered with any vehicle).
