Driver under 21 and km crossing at Huk Coburg

Hello, my car is insured with the HUK-Coburg. And my mother is the vehicle owner and policyholder, so she runs on your name and of course she specified that I, so a driver under 21 is driving with it. Is that directly tied to me as a person? That means what if one of my buddies (also under 21 and of course in possession of a driver’s license) drives (for example during the upcoming holiday) and what happens? Does the insurance grip even if he has driven and And then another question the car is given with 9000km/year. What if I exceed this? Or is there a regulation that says up to 25% about it is still ok or similar? Would I be happy about answers and sorry if these questions already existed. PS: Of course you always hope that nothing will happen, but you never know!