I will try to explain how child themes work. Basically you will have all the original theme files in a separate folder which we call it as parent theme. In this example, _brand.php file is already there on the original theme (parent theme).
To extend it’s functionality, we’re setting up a child theme and copy the template file into the child theme. We’re doing that because, if we directly edit the original theme files as you have suggested, it will be overwrite every-time when you update X theme.
To keep the customizations separately, you need to setup a child theme. If you want more information, I would suggest you to read the following article (http://www.wpbeginner.com/beginners-guide/wordpress-child-theme-pros-cons/).
Hope that’s clear.