By default, Excel uses the same calculation mode for all open workbooks.
The Calculation mode is initially set from the first workbook opened and is not changed when other workbooks are opened.

Excel uses the same calculation mode for all open workbooks.

Excel sets the initial calculation mode from the first (previously saved) workbook opened. This initial calculation mode will not change when additional workbooks are opened, and will only change when either the user or a VBA program changes it. If you start Excel with a brand-new empty workbook and make changes to that workbook then the calculation mode is Automatic.

If a manual mode workbook is opened in automatic mode the workbook will be calculated.