I almost didn't buy the great book The One Billion Dollar App because of hits clickbait title, but it actually well-elaborates the mechanics (and the mathematics) of viral spread of apps, which not by coincident corresponds to the familiar formulas that people will have seen during the CoViD19 pandemic ("r-coefficient", r or R0 [1]).
For example, if you have a mobile app that gives you something free for each friend you invite to it, it may encourage some folks to share with r friends...