The position the featured image appears in is controlled by the theme’s design and layout. To change this you’d need to customize the theme by editing theme files and code.
Concerning the image displayed there, the theme normally displays the post assigned featured image. When a featured image is not set and the theme’s autoselect images option is enabled, the first associated content image will be used. Note that this is not necessarily the first image placed in the content (or even an image that’s still in the content).