When using free spins, money is not debited from the principal balance. All winnings are credited to the bonus account. After the wagering requirements are met, the money can be sent for withdrawal. Free spins, like other casino bonuses, are subject to certain conditions.
All in all, there are free slots that pay real money, but you will have to be playing at real money online casinos as opposed to social casinos on slot apps, or playing titles in demo mode.