iOS Development: How can I prevent an iPad from running a universal app in iPad mode?

Question

I'm diving into iOS development and I created a universal app that turned into an iPhone-only app. When it runs on the iPad, it just loads a white screen since there's no iPad code written yet. What I'd like is for it to run in "iPhone" mode on the iPad, if it somehow ends up on an iPad. I have the "Targeted Device Family" property set to "iPhone", so that should prevent it from showing up in the App Store as an iPad app, but if anyone owns both an iPad and an iPhone, then the app could end up synced to the iPad, at which point it will just load the white screen because it will try to run the app in iPad mode, which it doesn't have any code to support. In this situation, I prefer that it actually ran on the iPad, but in iPhone mode.

My questions are...

1. When an iPad runs a universal app, how does it know to run it in "iPhone mode" or execute the iPad specific code?
2. In a universal app, how does it know which code is iPhone and which code is iPad?
3. How can I prevent the iPad from trying to run the iPad code and, instead, run the iPhone code?

I apologize if I sound like a total noob, but I am. Thanks so much for your wisdom!