A Windmill microloan is considered an installment loan product. You are required, per your repayment schedule, to make monthly payments on your loan until it is repaid, based on the amount that was disbursed to you. When you log in to the Client Portal, you will see the Outstanding Balance of your loan, including all interest and principal that is owing.
Interest is charged on your loan daily on the amount that you have borrowed against the length of the loan term. You will see the cost of borrowing reflected in the Loan Disclosure Statement you have signed. If your loan is restructured (ie. you ask for an extension of lower payments), the cost of borrowing will change. For example, if the term of your loan is lengthened, the cost of borrowing will increase, as you will be repaying the loan over a longer period of time and therefore be paying more interest on the amount you borrow.
When a payment from your account is received, it is applied first to any outstanding fees, then any outstanding interest, then any outstanding principal. Any remaining amount will be applied directly to your outstanding principal.
For example, if you miss a payment on your loan, a deposit will first go towards recovering the missed payment.
If you are interested in learning more about installment loans, refer to this Investopedia article.