When you're financing a purchase, whether a car, a home, or anything else, the number that actually matters is your monthly payment, and that depends on how much you're borrowing after your down payment, not the sticker price. This tool takes the total price and your down payment (as a dollar amount or a percentage) and works out the monthly payment, total interest, and total cost from there.
Whichever is easier for you to think in. Both give the same result: switching between $ and % just changes how you enter the same underlying amount.
The total price of the purchase plus all the interest you'll pay over the life of the loan. It doesn't include fees, insurance, or taxes, since those vary too much by lender and location to calculate universally.
Yes, the calculation works the same way for any amortizing loan, whether it's a home, a car, or anything else financed with a fixed monthly payment.
A larger down payment means a smaller loan amount, which means less interest accrues over the life of the loan. The savings come from both the reduced principal and the interest that principal would have accumulated.