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 NDJSON?
NDJSON is one JSON object per line, and nothing wrapping them. That single change makes it streamable - a program can handle a line at a time instead of holding the whole array in memory - which is why logs, data exports and machine-learning datasets ship in it.