I think the greater achievement came with Google image search, someone had to tag all those photos.
They wrote an algorithm that takes that data and recognises new images with it. As long as there is a way for us to tag inaccurate matches then it should be able to continue to learn. I imagine any flagged matches are being reviewed carefully.
I always thought that was done using keywords on the page the image was taken from (and image captions, alt text, titles and filenames). This is reinforced by what you see when you go many pages ahead in images and see things that don't seem related to what you searched, but the keyword is there somewhere on the page.
They wrote an algorithm that takes that data and recognises new images with it. As long as there is a way for us to tag inaccurate matches then it should be able to continue to learn. I imagine any flagged matches are being reviewed carefully.