The `Spreadsheet` class is the core of PhpSpreadsheet. It contains references to the contained worksheets, document security settings and document meta data.
Typically, you will create a workbook in one of two ways, either by loading it from a spreadsheet file, or creating it manually. A third option, though less commonly used, is cloning an existing workbook that has been created using one of the previous two methods.
Details of the different spreadsheet formats supported, and the options available to read them into a Spreadsheet object are described fully in the PhpSpreadsheet User Documentation - Reading Spreadsheet Files document.