The block editor is a single-page JavaScript application that happens to be integrated into WordPress admin. While post.php is still involved, it only bootstraps the JavaScript application rather than handling the interface.
This architecture means the block editor could run completely outside WordPress, as demonstrated by the standalone Gutenberg demo. Everything on screen is interconnected – sidebars, content, and controls all communicate as one unified application.
Unlike the classic editor’s readable PHP source code, the block editor uses compiled JavaScript that requires development tools to modify or understand.