How do apps make money if they are free to download?

I've often wondered, how do apps that are freely available to download actually generate revenue? Is it through in-app purchases? Do they rely on advertising? Or perhaps there's a more sophisticated monetization strategy at play? I'm genuinely curious about the inner workings of this digital economy and how developers are able to sustain their creations without upfront costs to the end-user. Can you enlighten me on this topic?