I was able to locate closing_popup.js at: \addons\point_of_sale\static\src\app\navbar\closing_popup.js\closing_popup.js
but I don't think that's the file, I previuosly modified report_userlabel.xml and report_sale_details.py to get some custom field and it worked. Can you provide the details on what to add to the closing_popup.js?

In the closing pop up file there is a function named downloadsalereport inside that a report is called that report is used to download that as pdf
what you need to do is create a report for both xlsx and csv then inherit downloadsalereport this function and chage the function to download either csv or xlsx

on the other hand hide the pdf download button and add two new buttons for csv and xlsx on the onclick function navigate to respective created reports