There are several ways to monetize games. I personally have no experience with any of them, so please take my advice with a grain of salt :wink:
Here's a few potential ways to earn revenue from free games (that I can think of):
Paid DLC/Expansions (Additional areas, characters, story, game modes, etc)
IAPS (In app purchases. Similar to DLC/Expansions, but are generally just consumable items and/or cosmetic items)
Ads (Overlay adds, banner adds, video adds, etc)
Release a demo of the game, and charge for the full game
Ask for donations (with Itch.io you can set a suggested donation amount)
A service like Patreon. Have patrons gain access to your games earlier then everyone else, or some other incentive (some examples: ability to watch you stream development, vote on features, QA sessions, access to the game assets, tutorials)
* Find a sponsor (I have no idea how feasible this one is)