When you make the required monthly minimum payment, you are paying towards both the principal (loan amount) and the interest added.

At the beginning, the majority of the payment is going towards the interest and then the ratio slowly changes so that towards the end, it is the principal that is what most of the payment is for.

When the interest rates change, they look at how much principal is left and then redo the payment amount with the new percentage and years left. So, if you have $65,000 left and you've been paying for 5 years, then it's the same math as a $65,000 loan on a 25yr repayment. So look at how much principal is left on the loan and then the time left on the 30 years.

Now, your monthly payments are sometimes locked in until you hit a "trigger" where the monthly payments need to be adjusted. If it isn't adjusted, then you keep paying the same monthly but then have balloon payments or longer payoff length (more than 30 years).