Besides getting businesses to advertise on the mobile app, how do these free mobile apps actually generate revenue? Why would any business pay to be advertised on a mobile app that just started out and has no traction as of yet? In addition, how much revenue does such a mobile app usually generate in a month? App category that I'm referring to: - gaming - travel - social
100s of ways.
Start by reading the book "Free" by Chris Anderson for a history of old style + new style approaches.
Then hire a bunch of smart people for an hour each to tell you how they'd monetize free for your project.
In fact searching for - how to monetize free apps - returns 25M results for me, so likely this search might be a good starting point for you.
Also study Podcast Monetization + Radio Show Monetization, both of which have been monetizing audience free content for years.