The classic editor is built on traditional PHP foundations. The edit post screen is essentially one large PHP script (post.php) that handles all functionality server-side.
JavaScript is optional and only enhances the user experience through AJAX requests. The editor works perfectly fine with JavaScript disabled because it’s fundamentally a PHP form submission system.
TinyMCE provides the rich text editing experience as an independent JavaScript application that has no relationship to other page elements. This isolation allows multiple editors on the same page.