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 JSON?
JSON is the shape data arrives in when it comes from an API. Tabular data usually lands as an array of objects, one per row, with the column names repeated as keys in every single one - verbose to read, but unambiguous.