Photos.app uses non-destructive edits, meaning that yes, essentially a set of instructions for applying edits is stored alongside the original bitmap, as opposed to duplicating the bitmap with the edits applied. Most likely it is stored as metadata within the file as opposed to a separate file. You can revert to the original at any time.
You can also duplicate the image as a kind of "snapshot" - any edits made to the point of duplication will be applied to the duplicate image (again, non-destructively), but both the initial image and the duplicated image are still pointing to the same bitmap on disk. Any further edits made to either image will not be reflected in the opposite image, and you can revert either to the unedited original at any time.