A child theme is a theme that inherits the functionality and styling of another theme, called the parent theme. Child themes are the recommended way of modifying an existing theme. When a theme update releases, you can update it without fearing that your code customizations will be lost.
No, if you don't need to edit the code or add snippets, installing the child theme is optional
You can find the child theme in themeforest downloads section.
Same as the parent theme, go to appearance > themes > add new > upload the my-listing-child.zip and activate it. Remember that when doing code customizations, the child theme must be activated for the changes to take effect