The WordPress cron is run by the
wp_cron() function, which is hooked to run on the
init hook, which runs on every page load.
wp_cron() is defined in
wp-includes/cron.php and hooked in
wp_cron() function kicks off a
wp_remote_post() request to
/wp-cron.php. Some server configurations prevent scripts sending a request to the same domain like this however, so as an alternative you can set the
ALTERNATE_WP_CRON constant to
true. When enabled this redirects the user to the current URL but with
?doing_wp_cron= added to the URL, instead of the post request.