Car accident, insurance question

My little brother who is 18 and recently got his license, was in a car accident today. He is at fault. The person in the other vehicle was flown to the nearest hospital (a town away). I don’t know the extent of the persons injuries at all. My aunt went to the scene and the firefighter told her they were flying him as a precaution. Thankfully the man is alive and okay as far as we know. My brother has liability only so I’d imagine that there is no way in hell that his insurance will cover the bodily injury costs in full.

Does anyone have any experience with this? I know that he would be responsible for the difference in medical costs after insurance liability coverage. He doesn’t own anything or have any assets so besides wage garnishment, I’m not sure what happens. I’m also not sure how wage garnishment works. Do they tell him that he owes $100k (hypothetically) and then work out a mandatory payment from his paychecks? Or do they take the whole paycheck until it’s paid off???? I really know nothing about this.

PS we live in California

EDIT: He was not on his cell phone. He has poor drivers experience and just moved to an unfamiliar area. We grew up in a small town. Straight back roads, basic freeways. He was in an accident on Highway 4 which is a very dangerous highway in CA. He took a turn to fast and hit the oncoming car on the drivers side. I’m asking very clear questions. Obviously he made a mistake and it was an accident. If you can’t contribute then keep scrolling.