Google offers Day wise money to Download this app
The way a developer gets revenue from free apps is usually through advertisements and/or in-app-purchases. On Android, there are many choices for ad libraries and networks.
AdMob is one by Google. Some say that it earns very little compared to other networks. AdMob alternatives can be found and compared by online searches.
Another one that I have looked at is Enhance™ by FGL.com which says it implements all ad networks and chooses the most profitable one. It can insert ads in an uploaded app without any code being written.
In-app-purchases are another way for free games to make money. A common strategy is to have an app with ads and then an IAP to remove the ads. Another method is to make some features "premium" and only available after a one time purchase or subscription.
With my free app Indefinite: Interrogation Game, I used both ads and IAPs. In about 2 years it got over 200000 installs in and earned around $1500. Although it was called an interesting game and is highly rated, most users stopped playing it after a day or two. With few users actively playing the game, revenue isn’t high.
