How do I add CSS & JavaScript to ?
On you THEME.libraries.yml:

global-styling:
  version: VERSION
  header: true // &lt;--- To the header!!!
  css:
    theme:
      css/mystyle.css: {} // if you want add custom css
  js:
    js/stepcarousel.js: {}
  dependencies:
    - core/drupal // for use drupal.js
    - core/jquery


On you THEME.info.yml:

name: THEME
base theme: Bartik
...
libraries:
  - THEME/global-styling
...


and Clear Cache


  Where is page.tpl?


Here: core/modules/system/templates/page.html.twig

If you want use it, copy to theme folder inside templates, like this:

 THEME
   templates
     page.html.twig


Change all you want and Clear Cache