Nested Paragraphs + PL

1. flexible-grid.twig

{#
 # This is the PL template for the "parent" pattern
#}

{% set flexible_grid_base_class = flexible_grid_base_class|default('flexible-grid') %}
{% set item_count = flexible_grid_item_count|default(rows|length) %}

{# Dynamically define grid columns #}
{% if item_count is divisible by(3) %}
  {% set flexible_grid_modifiers = ["columns-3"] %}
{% elseif item_count is divisible by(2) %}
  {% set flexible_grid_modifiers = ["columns-2"] %}
{% endif %}

<div {{ bem('wrapper', [page_layout_modifier], flexible_grid_base_class) }}>
  {# Heading #}
  {% if flexible_grid_heading %}
    {% include "@molecules/headings/section-heading.twig" with {
      section_heading: flexible_grid_heading,
    } %}
  {% endif %}
  <div {{ bem('content', flexible_grid_modifiers, flexible_grid_base_class) }}>
    {# Content #}
    {% block flexible_grid_content %}
      {% for item in rows %}
        {% include "@organisms/flexible-grid/_flexible-grid-examples.twig" %}
      {% endfor %}
    {% endblock %}
  </div>
  {# Link #}
  {% if flexible_grid_link_url %}
    {% include "@atoms/00-links/link/link.twig" with {
      link_url: flexible_grid_link_url,
      link_content: flexible_grid_link_text,
      link_blockname: flexible_grid_base_class,
      link_modifiers: ['more', flexible_grid_button_color],
    } %}
  {% endif %}
</div>

2. paragraph--testimonial-grid.html.twig

{#
 # This is the Drupal template for the "parent" paragraph type
#}

{% embed "@organisms/flexible-grid/flexible-grid.twig" with {
  flexible_grid_heading: content.field_headline,
  flexible_grid_link_url: content.field_link.0['#url'],
  flexible_grid_link_text: content.field_link.0['#title'],
  flexible_grid_button_color: "blue",
} %}
  {% block flexible_grid_content %}
    {{ content.field_testimonials }}
  {% endblock %}
{% endembed %}

3. testimonial.twig

{#
 # This is the PL template for the "child" pattern
#}

{% set testimonial_base_class = testimonial_base_class|default('testimonial') %}

{% set testimonial_image %}
  {% block testimonial_image %}
    {% include "@atoms/images/image/_image.twig" with {
      image_src: testimonial_image,
    } %}
  {% endblock %}
{% endset %}

<div {{ bem(testimonial_base_class, testimonial_modifiers, testimonial_blockname, ['flexible-grid__item']) }}>
  {% include "@atoms/01-text/text/blockquote.twig" with {
    blockquote_content: testimonial_content,
  } %}
  {% embed "@atoms/00-links/link/link.twig" with {
    link_base_class: 'image',
    link_blockname: testimonial_base_class,
    link_url: testimonial_link_url,
  } %}
    {% block link_content %}
      {{ testimonial_image }}
    {% endblock %}
  {% endembed %}
  {% embed "@atoms/00-links/link/link.twig" with {
    link_base_class: 'name',
    link_blockname: testimonial_base_class,
    link_url: testimonial_link_url,
  } %}
    {% block link_content %}
      {% include "@atoms/01-text/text/text.twig" with {
        text_content: testimonial_name,
        text_base_class: 'name',
        text_blockname: testimonial_base_class,
      } %}
    {% endblock %}
  {% endembed %}
  {% include "@atoms/01-text/text/text.twig" with {
    text_content: testimonial_title,
    text_base_class: 'title',
    text_blockname: testimonial_base_class,
  } %}
</div>

4. paragraph--testimonial.html.twig

{#
 # This is the Drupal template for the "child" paragraph type
#}

{% embed "@molecules/testimonial/testimonial.twig" with {
  testimonial_content: content.field_simple_text,
  testimonial_name: content.field_attribution_name,
  testimonial_title: content.field_attribution_title,
  testimonial_link_url: content.field_link.0['#url'],
} %}
  {% block testimonial_image %}
    {{ content.field_image }}
  {% endblock %}
{% endembed %}

"There’s a lot going on in that example, but the basic idea is that the “parent” pattern has a twig block, and you just pass in the content.my_actual_content_field field into that block in the Drupal template."
– bjlewis2