Add sections, components and blocks to Layout Builder programmatically

In the case that you have a field on a node that you have some data in, and you wish to add this to the layout builder of the page programmatically, here is how to do so, with a landing page type node that has a field called 'field_accordion':

use Drupal\node\Entity\Node;
use Drupal\layout_builder\Plugin\SectionStorage\OverridesSectionStorage;
use Drupal\layout_builder\Section;
use Drupal\layout_builder\SectionComponent;
use Drupal\layout_builder\SectionListTrait;

// First load your node
$nid = '441';
$node = \Drupal::entityTypeManager()->getStorage('node')->load($nid);

// Get sections of layout builder
$layoutBuilder = $node->get(OverridesSectionStorage::FIELD_NAME);
$sections = $layoutBuilder->getSections();
$section = $sections[0];

// Add field of node to layout builder, in this case, the field is 'field_accordion' that already exists in the node


        $newBlockConfig = [

          "id" => "field_block:node:landing_page:field_accordion",
          "label" => "Accordion",
          "label_display" => 0,
          "provider" => "layout_builder",
          "formatter" => [
            "label" => "above",
            "type" => "entity_reference_revisions_entity_view",
            "settings" => [
              "view_mode" => "default",
            ],
            "third_party_settings" => [],
          ],
          "context_mapping" => [
            "entity" => "layout_builder.entity",
            "view_mode" => "view_mode",
          ],
        ];

  // Create a new section component using the node and plugin config.
  $component = new SectionComponent($node->uuid(), 'second', $newBlockConfig);

  // Add the component to the section.
  $section->appendComponent($component);

  // Set the sections.
  $node->layout_builder__layout->setValue($sections);

  // Save the node.
  $node->save();