Image Magic!

Ok why is it that when i load the theme twenty fifteen, it seems no matter what the content of the post displayed is on the home screen It manages to find an image Whether its a embed youtube video a embedly page anything it seems if there is a image to be had it finds it. This doesnt seem to happen on other themes you have to frig around with the featured image install plugins that create featured images etc etc.

So my question how is this so? Is there a easy way to get this function in other themes? I have no idea why it is not standard.
What is the function? called?