SpreadsheetML
First of all, Excel files (.xlsx) are effectivly just a zip archive that contains several different XML files. These XML files contain the actual data of the workbook, information on the worksheets and tables within worksheets and stylings, pivot tables and much more. There is a standard structure for this called Open Office XML - SpreadsheetML (OOXML).
Here is a summary of the general structure of an excel file.
Here is another screenshot from ECMA-376 about the minimum file structure for an XLSX file with three worksheets.
There are some extra folders and files that may exist inside any document that falls under the OOXML file structure (.pptx and .docx).
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/xl/media/--> Contains image files (.jpeg, .gif, .png and .pict). -
/customXml/--> Contains most of the shared XML parts with items like SVG markup for drawings or item definitions.