Using it

From any Twig template in your theme, use include with the namespaced name. The namespace is your theme's machine name:

twig
{{ include('mytheme:test-card', { heading: 'Hello' }, with_context = false) }}

For slots, use embed:

twig
{% embed 'mytheme:test-card' with { heading: 'Hello' } %}
  {% block body %}
    <p>Some content here.</p>
  {% endblock %}
{% endembed %}