Using last versions of woocommerce plugins IS MUST! Plugin created according to the problem: https://qna.habr.com/q/1336850. In short last version of the plugin uses Gutenberg block instead of basic php files, and if we want to set our custom rules we need to make a little bit of new changes. Our goal is to customize checkout from the custom plugin, so first of all, we need to disable the Guttenberg form blocks, and then set shortcode, from this part of the moment we can customize the form as basic by hooks and changing files inside checkout folder, of course, we shouldn't make manipulations inside woocommerce plugin folder, just copy files which we need to change inside our custom plugin folder, and add connection template logic to the plugin, of course, if you need this functionality.