The scheduled payment is the amount that would finish the loan on the original term. Interest each month is the remaining balance times the monthly rate, rounded to the cent. The payment reduces the balance. The second schedule adds the extra dollars to every payment.
A $10,000 loan at 6% for 3 years has a $304.22 payment and $951.88 of interest over 36 months. Adding $50 a month finishes in 31 months and cuts the interest to $807.29, which saves $144.59. A lender that applies the extra on a different day, or that charges a prepayment fee, will not match this schedule.