The second image is a raster layer and in the third image obtained vector layer is so accurate.
What algorithm would have been employed?
Normal methods doesn't give nearly equal vectorization.
I applied Raster to polygon conversion and the output is the below image where the green=raster, red=accurate vector,black=result of raster to poly. the red and black both are vectors but are very different.
Green line original raster, red is the accurate vector, Black is the outcome of the raster to polygon func.