What is DOCX?
What Word has saved since 2007: a zip of XML parts, replacing the binary .doc that came before it. It is a page format through and through - paragraphs, styles, revision marks - with no notion of a typed value anywhere in it, so a table in a Word file is text arranged in a grid rather than data.
What is XML?
Verbose, strict, and still holding up enormous amounts of the world - government feeds, financial messaging, publishing, office documents. JSON took the web from it, but anywhere with schemas and validators and twenty years of history, XML is what's on the wire.