I should also state that the directory structure and files are for the default theme. If you have a child theme then you would want to put the /SO9/_software/template/single-listing.php file directly into your child theme directory.
If it helps… whenever I use a child theme I typically override a couple files to make it use my child theme directory files.
I’ll only copy the core files/templates I need into my child theme directory and make changes on those. This way, when there is an update and the site breaks, I stop the overrides to revert back to default theme until I can fix it. Then I only update what’s needed within my customization before overriding the files again.