Include template
Many developers prefer to store header/footer code in a separate file and include that file in page.html.twig.
Process
Let’s say you’ve created the following file in your theme’s folder for the header:
THEME_NAME/templates/includes/header.html.twig
And now you want to include this file in:
page.html.twig
Recommended Method
The proper method for Drupal 8 themes is to use Twig namespaces to reference the current theme’s "templates" directory. Here’s an example:
{% include '@THEME_NAME/includes/header.html.twig' %}
Below is an example from a working theme on Drupal.org called Architect.
“@architect” refers to the /templates directory inside the working theme (architect).
{% include '@architect/includes/header.html.twig' %}
The Twig namespace is created automatically in Drupal 8 when your theme is installed and points to your theme’s /templates directory. Essentially, writing “@theme_name” in a Twig include (as shown above) will reference the location your_site.com/themes/theme_name/templates on your server.
Not Recommended Method
One possible (but not recommended) method is to use the following code to include the file.
{# NOT recommended #}
{% include directory ~ '/templates/includes/header.html.twig' %}
The above may work, but it will result in a server error when used with a subtheme.
Extending Namespace Functionality
The Component Libraries module provides more flexible and advanced organization of Twig templates in Drupal 8, which can work well with Twig's "embed" functionality.