core/jquery not working

Hi, I am very new to drupal and I need to call jquery.

I added this to mytheme.libraries.yml:

mysubtheme:
  css:
    theme:
      css/slippry.css: {}
    header: true
    js:
      js/slippry.js: {}
  dependencies:
      - core/jquery

All tutorials say this should call jQuery but for me it does not. The only way I can get my jQuery to work is if I put the script src=//jquery-url to my page.html.twig file, and I do not want to do this as jquery should be loaded client side.

Does anyone have an answer to this?

Drupal version: 


Source: https://www.drupal.org/taxonomy/term/3/feed