How do apps connect to the internet?

[AC Question]

If I download an app for a hotel chain, airline, bank, etc. to my Android phone, and I activate that app, how does the app connect to the internet to gather data? How does the app choose between wifi and a browser? If there is no wifi, does the app automatically invoke a browser on my phone? I'm considering a Tracfone Samsung Galaxy Core Prime phone, but the question seems generic to many Android apps.