Here's how your loan and payment work.

You take your principal amount (or loan amount) = P

Take your yearly interest rate (probably around 4%-10% depending on credit) = I

Now most interest is calculated on a monthly basis, which is 1/12th of I = I/12

At the end of each month you take the current principle amount and multiply it by your monthly interest rate = P * I /12

This number is how much you owe in interest at the end of the month. We then repeat this process every single month. As your principal decreases you interest payment at the end of the month will also decrease.

When you pay your $370 you will first pay off the interest we just calculated. Then any remaining amount will go towards the principle. This is great because then next month the principle will be lower, meaning you'll owe less in interest, which allows more of your $370 will go towards the principle.

If you paid $400 each month, the extra $30 would go towards the principle since there is (almost always) no such thing as "prepaying" interest. You only owe interest at the end of the month and you only owe it for the month that just happened. You won't pay $30 today and have that go to some other month's interest amount.