Is there a plugin or another way that can help me find out the underlying files of a page/post? [closed]

For the template, you can use Query Monitor:

  • Activate the plugin and browse the site logged in
  • Click the new timings numbers on the admin bar at the top to bring up the bottom pane
  • Select ‘Template’ on the left menu

(or probably other plugins too)

For the CSS files your best bet is to use your browser’s debug tools, the Network tab, to see what’s loaded.