What is JSON?
The format the web runs on. Nested objects and arrays, real types, and a syntax small enough to fit on a postcard. It maps badly onto a grid, which is why converting deeply nested JSON to a spreadsheet always involves losing something.
What is XLSX?
A modern Excel workbook, which is really a zip full of XML. Unlike a CSV it knows what a number is, holds multiple sheets, and keeps formulas and formatting. It's what people mean when they say 'send me a spreadsheet'.