I don't upscale images, but my first thought is ImageMagick. Googling "upscale imagemagick" seemed to indicate that it is suitable for the task.
More generally, upscaling is probably going to be >95% established algorithms with at best a small amount of perceptual heuristics to accommodate human vision. But I wouldn't expect perceptual heuristics to be common outside of tools like Photoshop because most of the time it won't matter and SEO is going to generate more revenue for a web service than quality.
I doubt there's much proprietary technology, it's understanding graphics sufficiently to make good judgements that's going to be rare.
I'm not exactly sure how these sites work, but they're clearly doing something different from "regular" upscaling like what happens when you just make a picture larger and blurrier, it looks like some kind of semi-vector type conversion before the resizing, but it's never described beyond the regular AI/ML buzzwords.
The reason I ask is because it could be really useful for stuff like wallpaper resizing where the only thing you really want is for the larger size to look decent, not necessarily be exactly like the original.
More generally, upscaling is probably going to be >95% established algorithms with at best a small amount of perceptual heuristics to accommodate human vision. But I wouldn't expect perceptual heuristics to be common outside of tools like Photoshop because most of the time it won't matter and SEO is going to generate more revenue for a web service than quality.
I doubt there's much proprietary technology, it's understanding graphics sufficiently to make good judgements that's going to be rare.