Android Navigate Up mechanism

Question

In hierarchical navigation, the user navigates up by pressing the Up button on the left side of the toolbar. As of Jelly Bean, there is a easy way to achieve this functionality. Enable hierarchical navigation by adding a parentActivityName attribute in the AndroidManifest.xml file.

When the user navigates up from ChildActivity, an intent like the following is created:

Intent intent = new Intent(this, ParentActivity.class);
startActivity(intent);
finish();

I want to know what exactly happens behind the scene, especially what the intent contains?

Moreover, when I want to preserve the parent activity's state. I know that it can use addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP). Like Android: navigating up to the parent activity recreates the parent activity The first answer says that the intent contains Intent.FLAG_ACTIVITY_CLEAR_TOP . The standard behaviour of this flag is to finish all Activities that are on top of the parent Activity in the task stack including the parent Activity itself and then launch a new instance of the parent Activity.

But when I add the following code in the ChildActivity, running it in the device prior API 23 ,the ParentActivity will not be destroyed and the system just resume the old parent activity. While running it in the device with API 23, the ParentActivity will be destroyed and the system instantiate a new ParentActivity. But in both cases the ParentActivity will preserve its state. What is going on?

public Intent getParentActivityIntent() {
    return super.getParentActivityIntent()
            .addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
}

and where I can find the source code that achieve the UP function! I had google search, but could't find the answer I need! Thanks in advance!