This file can be anything really, but in its simplest form it’s a shell script. Inside the MacOS directory, there’s an extension-less file with the exact same name as the app itself. The internal folder structure may vary between apps, but you can be sure that every Mac app will have a Contents folder with a MacOS subfolder in it. You can view the application’s contents by navigating to it in the Finder, right-clicking it and then choosing “Show Package Contents”. app extension, but it’s not really a file - it’s a package.