The interest is charged on the next day for the previous one. For example, you borrowed 100 USDT on Monday. The interest will be charged at night on Tuesday. Important to note! When you repay the loan, the interest is charged for the day, when you repay the loan as well.
The basis to calculate the size of the interests is set to the loan amount (incl. its changes during the day).
Because there's no fixed date when to repay the loan, as well as the loan amount can be changed at any time from your side, we use a compound interest formula to calculate daily interest.