Need help with how custom fields are displayed by article.html.twig file
If I understand correctly:

The node.createdtime in the screenshot comes from the content type setting "Display author and date information" on the content type edit page. Look at /admin/structure/types/manage/article.

If you need a different date shown, you can uncheck that setting to hide Drupal's author and date info display, then ensure your custom fields are displayed on the node instead. Look at /admin/structure/types/manage/article/display.

If the template node--article.html.twig renders content with just {{ content }}, your fields will show once set to display. If your template file is customized, you may need to add {{ content.field_publicationdate }}.

As @Kevin mentioned, use the field format options at /admin/structure/types/manage/article/display to control the output of your date field.



A more technical approach, and if your content is being imported, could be to map your field_publicationdate value to the node created time value. (But that's another question :)