If you are trying to get the single store pages to display or override a unique single page template (single-stores.php), as opposed to the standard single.php template for blog posts.
You can create a single-stores.php in your child theme. This will override the normal template.
When using the single-stores.php, is there a way to extend or modify the output of stores_single_content in the content? Such as creating a customized stores_single_content function in functions.php or via another method.
You can hook into the “the_content” function yourself as we do.
Which folder should I install the single-stores.php in the child theme?
You can put the template file directly into the main folder.