Free apps make money with Ads, maybe I need to create a guide for that, but it's very understandable, if you're using a free app, let's take face book for example, it allows you to connect with your friends and family and watch videos, basically anything social.
But in turn they sell your information to advertisers as data to make money, that's why sometimes you'll see sponsored ads on your feed and in someway you can relate with that ads, that's because of what you've been searching, I hope you understand, and you shouldn't underestimate the advertising industry because it's a billion dollar industry.