Create a Paragraph or Nested Paragraph and Add to Node

Create a paragraph

use Drupal\node\Entity\Node;
use Drupal\paragraphs\Entity\Paragraph;         

$paragraph_1 = Paragraph::create([
    'type' => 'paragraph_type_1', //machine name of paragraph
    'field_text' => [
      'value' => 'Some text',
      'format' => 'full_html',
    ],
]);
        
$paragraph_1->save();

Create a nested paragraph

  

$paragraph_2 = Paragraph::create([
    'type' => 'paragraph_type_2', //machine name of paragraph
    'field_text' => [
      'value' => 'Jesus Saves',
      'format' => 'full_html',
    ],
    'field_paragraph_reference' => $paragraph_1,
]);
        
$paragraph_2->save();

$node->field_content->appendItem($paragraph_2);
$node->save();