Expand all the messages in this[^1] Google support thread, and note the comments from "Craig[^2]."
Or, even better: open a .gsheets or any .gEXT file in your favorite text editor. You'll see that they are comprised of web markup language, much like an XML doc. I don't have one in front of me atm, but I recall thinking, "it's basically XML that points one's browser to the appropriate Google app and a unique ID for the document."