Whether you're playing real casino games or free casino games, the RTPs (return to player percentages) are fixed. There is nothing that can be done to change the house edge in casino games. Casinos do not intentionally manipulate demo games to payout more frequently than real money casino games.
Many online casinos offer a "Play for Fun" or "Demo" mode that allows users to try out slot games without risking real money. In demo mode, you use virtual credits provided by the casino to play the games.