Car insurance through the parents

Hello people I’m happy to be a member of here Have already read about the car insurance thing about the e.g. father in many forums, now to my question. I want to get a car this year and since I’m a beginner (18J.) I want to insure about my father, but do I have to be registered as a co-user with the insurance? Or is it simply stated that people under 21 drive the car too? Because I mean it’s the car insured and if I e.g. . the car of an acquaintance would I be insured as well? And sink in time also my percentages ? Many thanks in advance in greeting mcfly_GT “And a Happy New Year”