This is actually very simple. The interest will come from the already existing supply. How it should be.
The problem with increasing the supply solely through loans, like feat does, is that this creates a perpetual debt since there will never be enough money to pay for the total debt + total interests. This is because every loan comes with interests, which means that the only way to create the money necessary to pay for the total debt + interests, is through another loan, but this new loan also comes with interests, so the cycle literally never ends. Basically, this is a slavey model.
Bitcoin, on the other hand, only increases its supply through mining, and it stops its supply growth at 21 mil BTC. This makes it incredibly good as a store of value, but not that good for an everyday currency.
