To find out how an app makes money, look at its listing: the price, the « In-App Purchases » note and the privacy section. The model changes what the app expects from you.
Free — What it means: you pay nothing. · What to watch for: advertising, or the use of your data.
Freemium — What it means: the basics are free, the rest is paid. · What to watch for: useful features hidden behind a payment.
Paid — What it means: you pay once, or every month. · What to watch for: automatic renewal.
Provided by an organization — What it means: a clinic or an employer pays. · What to watch for: who sees your data.
Limitation: the store listing does not always show the price of the options. The app displays it at the moment of payment: nothing is charged before you confirm.
To picture it: an app has to earn its living somewhere. Knowing where helps you know what it is asking of you.