SheetCompareXLSX
The modern Excel file format: a ZIP archive of XML parts that stores values, formulas, formatting, and multiple sheets. The default save format since Excel 2007.
XLSX is the format Excel has saved by default since 2007. Despite the single extension, an .xlsx file is actually a ZIP archive: rename it to .zip, unzip it, and you'll find XML files for each worksheet plus shared strings, styles, and metadata. That structure is what lets it preserve formulas, number formats, charts, and several sheets in one file.
Because it is an open standard (ECMA-376), LibreOffice, Google Sheets, and dozens of libraries read and write it. The trade-offs show up when you compare workbooks: formula results can differ between calculation engines, cached values can be stale if a file was saved without recalculating, and dates are stored as serial numbers with a display format, so two cells showing the same date can hold different underlying values.