how to extends html files in jinja template engine code example

Example 1: how to declare and retrieve variable in jinja template

{% with my_variable="my value" %}
{{my_variable}}
{% endwith %}

{# or #}

{% firstof "my value" as my_variable %}
{{my_variable}}

Example 2: jinja2 template import html with as

{% from 'forms.html' import input with context %}
{% include 'header.html' without context %}