Hey CS Community- I'm a huge fan!

Just watched Grant's live feed of the fish and broth dish and at the end he finished it with walnut oil. I've heard a little about finishing oils but haven't found a good source that explains it well. Specifically I want to understand what they can be used in, how to use them, how to know what flavor oils will go with what, and how to know a cheap oil from a good one.
Does anyone have any good reading/videos on this or would anyone be willing to give a brief explanation?
Thanks!!!
Mike