How do we add columns in submission spreadsheet for internal use for notes

You have to add it as a field, you can add the text area field and hide it from the form view:

This hidden field will be visible if you edit the submission in the form submissions page. Here's how to view your form submissions page:

https://www.jotform.com/help/269-How-to-view-Submissions

To edit the submission, simply click "Edit" button in the page:

Or, you can integrate your from to Google Spreadsheet and add your notes in the sheet row. Here's how to integrate:

https://www.jotform.com/help/228-How-to-Integrate-Form-with-Google-Spreadsheet 

Thanks