It's the same way that google discovers them. The JSON+LD is on the recipe page. They place it there for search engines to discover it, so that they appear in a search result with the JSON+LD data: this data enhances the result, instead of just having the name of your recipe, the result contains description, image and ingredient lists.
You can see this in action on google search: type in something like "Chili soup", then it'll return the image, description, some parts of the recipe, etc.
You can see this in action on google search: type in something like "Chili soup", then it'll return the image, description, some parts of the recipe, etc.