I always thought stud exstracters were a bit of a gimmick til' I added some to my tool box. They come in various sizes & have a left hand thread so as you 'screw' them into the hole drilled into a snapped or stripped bolt it then winds the said bolt out of its right hand thread, if you catch my drift?
But get the dearest ones you can afford as the cheap ones also snap easily.
Good luck.