How does an app with lower base sdk work?

How does an app with lower base sdk work?

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 6, then how come the app has the old 'look and
feel'? Does XCode compile the older SDK and include it within my app or
does new version of iOS comes with older libraries/SDKs?
In other words, does the run time know this app is compiled with lower
base SDK and somewhere in UIKit's code it does:
if (lower SDK) {
//show old look/feel
} else {
//show new look/feel
}
or does the app itself include the old library and load it ?
Thanks