How are the apps published on the App Store licensed to end-users? What license is used by apps published on the iOS and Mac App Store? I am assuming this information is a part of the Apple Developer Program License Agreement.

Does the individual/organization publishing the app has a choice of license? Are the individual apps licensed as a part of license for iOS (the system software) itself?
According to the Apple Licensed Application End User Agreement:

Your license to each App Store Product that you obtain through the App Store Services is subject to your prior acceptance of this Licensed Application End User License Agreement (“Standard EULA”), and you agree that the terms of this Standard EULA will apply to each App Store Product that you license through the App Store Service

The Standard EULA license applies to any product that you obtain through the App Store unless:

Unless that App Store Product is covered by a valid end user license agreement between you and the Application Provider of that App Store Product, in which case the terms of that separate end user license agreement will govern.

That will be the case unless you have a separate user license agreement between you and the application provider.
In the article, it mentions nothing about choosing your own license.