How does instagress work?

Hey guys, since I was a kid I've always been aggressively curious. June 2016 Instagram restricted use of their API. How was instagress or other 3rd parties similar to instagress not affected? How are they able to collect data or call the API? Are they using the API? I have been tinkering around with various software like Followliker and other 3rd party tools for marketing automation. I'd like to understand what's happening inside the software or service?