The definition of aspect ratio is this: The ratio of the width to the height of an image or a screen. It makes no difference what the size of the picture is whatsoever. Aspect ratio refers only to the relationship of the width and height of the image itself. Watch a video below introducing what exactly Aspect ratio is and the differences between different Aspect Ratios.
https://nofilmschool.com/2015/04/vashi-visuals-high-resolution-aspect-ratio-templates