The original BLUERABBIT app is a WordPress theme - pages are WordPress pages, and every action runs through wp_ajax_* hooks. It works, but it ties the whole platform to WordPress internals it never actually needed.
The rebuild maps each of those pieces onto CodeIgniter 4 equivalents: singleton classes become Models and Services, AJAX actions become Controllers with JSON responses, and $wpdb queries become CI4's query builder. The result is a portable core - the same one this new site is built on.