Where you look, what you look at, how long you look, and where you ~~click~~ tap.

I'm a dev so can shed some light. It's likely linked to a service called Firebase, which gives many tools to track how people use your app. You can run tests against sections of your user base to determine the best way to apply certain features before rolling them out, as well as crash reporting and the ability to see "user journeys." These "journeys" are often used to determine how to better structure an app so that users can get to where they want to go more quickly. It's a way of collecting user feedback without asking for user feedback. It's incredibly useful for feature development and bug fixing

Now, given they have ads, they're probably using Google adsense which is likely linked to firebase, so Google may get some information on how people use the app in order to increase revenue per impression. This is largely automatic