How does a Genesis child theme understand that the Genesis theme is the parent theme?

I’m not experienced with Genesis, specifically, but I imagine its child themes work the same as child themes for any other theme. As documented, a child theme is created by adding a Template: line to the style.css header, which is the directory name of the parent theme:

/*
 Theme Name: Genesis Child Theme
 Template: genesis
*/

By defining the “Template” for a theme, WordPress functions like get_template_directory() know to use that theme’s directory for the path or URL.