how to add other component (html tag, js, and css) in form module
you should add a library like this :

$form['#attached']['library'][] = 'my_module/mylibrary';


Create mylibrary.libraries.yml file with the content :

mylibrary:
  version: 1.x
  css:
    theme:
      css/my-css.css: {}
  js:
    js/my-js.js: {}


more documentation: https://www.drupal.org/developing/api/8/assets