In Nebraska sales tax is on the payment so you dont pay the tax upfront like on a "buy" you would pay the sales tax each month i.e. pmt $300 tax 7%=$21.00 your pmt $321.00 with no up front tax
It depends entirely on the state (and sometimes on the local taxing bodies in the state). You cannot make any general statements on a national basis. For example, you would not pay sales tax on candy...
If you purchased the car from a dealership, the dealer collects it from you and pays it. If you purchased the car from a private owner, the county courthouse will collect it from you when you get it...