There is an important question because in general making revenue is the main goal of developing the application. There are many ways to make money with your mobile app. Let's see them.
Paid app. This is the first way. This is good for well-known apps when developers sure their product will buy. Also, you can make your app paid if this has a unique features that is not available in free alternatives.
Interesting statistic, that free apps make more revenue that free applications. The question is how? Let's see hoe to make money on free apps. There are some ways&
Advertisements. This is the simplest way to make money on your free app but you need to have some number of users to allow your app to be attractive for the advertisers. Also, you should be careful when implementing ads into your app because this can break down the UX.
Freemium. This name speaks for itself. Freemium means combining of free and paid features. Users can download a free app to understand if they need this app. And then they can buy a paid app with extra features. The main challenge is to keep balance between free and paid features.
More ways to get revenue from the free applications you can read in this article: https://themindstudios.com/blog/how-do-free-apps-make-money/