Xcode will build a framework for a functioning app with just a few keystrokes. Of course now one must become a developer, pay Apple a fee, and learn how to submit that free app to the App Store. That process can seem more complex than some apps are.
Are you talking about an app that shows a label saying 'hello world', or an app using advanced GPU accelerated neural networks to perform facial recognition of all of your photos?
Asking 'how hard is it to build an app' is like asking 'how hard is it to build a structure of some sort'? Well are you talking about building a small shack or a Burj-Khalifa scale skyscraper?