How do you differentiate between "stretch" and "upscale"? Or how would "stretching" by exactly 2x would make the image blurry?

Upscaling algorithms vary from the extremely basic to the ML models we see today that straight up replaces or adds new details. Some of the more naive algorithms do indeed just look blurry.

https://en.wikipedia.org/wiki/Comparison_gallery_of_image_sc...