100% financing means the whole cost of the car is loaned to you, you don’t need to put any money down. Those loans can come with onerous interest rates if you are at all late with a payment.
0% financing means they will not charge any interest for a period of time (usually one year). After that, you pay interest on the loan until it is paid off. Sometimes, when they start charging you interest, the interest has been accrued over the time of the “interest free period”
For instance, you buy a car for $20,000, paying $5,000 down and borrowing $15,000. If you pay the $15,000 over the course of the one year interest free period, you are done and never paid any interest.
However, if after a year you still owe 10,000, they start charging interest on the 10,000 balance, plus they may have the interest compounded from the start of the loan, so you get charged a year’s interest on that 10,000.