What is XLSX?
XLSX is what Excel has written since 2007 - not one file but a zipped folder of XML, with the sheet in one part and the strings in another. Its famous trap is dates: a date is stored as a plain number of days, and only the formatting attached to the cell says it is a date rather than a quantity.
What is XML?
XML is the verbose, strict ancestor of most structured data formats, and it is still what enterprise systems, publishing pipelines and older APIs exchange. Tabular data in XML is usually a repeating row element with one child per column.