How does an app with lower base sdk work?

Question

In XCode I can specify Base SDK. I am wondering how does that work behind the scenes? If I am running an app, for example, on a device that has iOS 7 and my base SDK is iOS

Answer 1

iOS applications are forward compatible with new versions of iOS. The reason is :

Almost all changes to the iOS versions are additive and hence an application build using lower version still runs on the higher iOS version.

Though, we need to take care of this point:

As frameworks evolve through various releases, APIs are introduced or deprecated and behaviors of existing APIs may occasionally change. Apple makes every effort to minimize changes that may cause incompatibilities, in some cases providing alternate behaviors based on the framework version. In rare cases, your code needs to determine the framework version and adjust accordingly

To understand more, read this