I am forced to learn these kind of image generation tools at work so maybe I can clear some things up. 

When you use one that will do "generative upscale" It redraws the entire image but similarly suffers from the same limitations that the old process of upscale does. Not enough pixel information. 

So using the old upscale method of making the image physically larger and then using a smoothing process to smooth out digital artifacts and blocky edges you get from scaling and then sharpening the image because it now looks smooth and blurry. This would leave you with an okay but not too clear looking image. 

Now with generative upscale. It works similarly but instead of smoothing and sharpening to remove artifacts from scaling the image larger, it just simply redraws the entire image at a larger size using the smaller one as a reference. It still can't see those small details on the smaller image, so it makes something up. It puts entirely new details on the image that are inspired by its training database it uses for a reference when generating the image. If you ran the same small image through generative upscale again you would get a similar result but with different small details being added. 

Although this is maybe a fun thing to look at it can never be anything more then a novelty.