Twenty Fourteen theme and the page-templates folder

I have to confess I’m struggling to see how the WordPress Twenty Fourteen theme picks up templates put in the theme’s page-templates folder. Previously I’d create new templates named page-<slug>.php in the theme’s root folder, but not now.

I’ve searched the code for mentions of page-templates and not found any where files are included; searched the WP source code too and found none.

Can you explain how they’re detected and picked up please?

The actual query for a Theme’s page templates happens in wp_get_theme()->get_page_templates() (source), which in turn calls get_files() (source).

The magic happens here:

$files = (array) $this->get_files( 'php', 1 );

The args for get_files() are:

function get_files( $type = null, $depth = 0, $search_parent = false ) {}

So, when WordPress searches the Theme for PHP files, it passes a $depth argument of 1, which means that it searches in subdirectories of the Theme root directory.