Numbers is Apple's spreadsheet app, paired with Pages and Keynote in iWork. Files are zip-packaged proprietary iWork XML. Core functionality overlaps with Excel but Numbers favors free-form layout (multiple tables per sheet, embedded charts, design templates) over Excel's grid-everywhere convention. Same compatibility caveats as Pages, convert to XLSX/CSV for non-Apple use.
How to open
Numbers on macOS/iOS, iCloud.com Numbers in any browser. Convert to XLSX or CSV for Excel/Google Sheets users.