Create a accordion plugin in wordpress

Install wordpress on your system

Create a directory named wp-multilevel-accordion in wp-content\plugins directory.

Create a file named wp-multilevel-accordion.php in wp-multilevel-accordion directory and copy/paste below code in it.

<?php
/**
 * Plugin Name: wp multilevel accordion
 * Description: A wordpress plugin for multi level accordion
 * Author: samar
 * Author URI: 
 */


if(!defined('ABSPATH')) {
    exit;
}

// Include necessary files.
require_once plugin_dir_path(__FILE__) . 'includes/post-type.php';
require_once plugin_dir_path(__FILE__) . 'includes/meta-box.php';
require_once plugin_dir_path(__FILE__) . 'includes/shortcode.php';


function my_plugin_enqueue_scripts() {
    // Enqueue stylesheet
    wp_enqueue_style('my-plugin-custom-style', plugins_url('bootstrap.min.css', __FILE__), array(), '1.0', 'all');

    // Enqueue script
    wp_enqueue_script('my-plugin-custom-script', plugins_url('bootstrap.min.js', __FILE__), array('jquery'), '1.0', true);
}
add_action('wp_enqueue_scripts', 'my_plugin_enqueue_scripts');

Create includes folder in wp-multilevel-accordion plugin directory and create file named meta-box.php , post-type.php and shortcode.php in it.

copy/paste code as per below.

wp-content\plugins\wp-multilevel-accordion\includes\meta-box.php

<?php
// Exit if accessed directly.
if (!defined('ABSPATH')) {
    exit;
}

// Add meta box for selecting parent accordion
function ml_accordion_meta_box() {
    add_meta_box('ml_accordion_parent_meta_box', __('Parent Accordion', 'textdomain'), 'ml_accordion_parent_meta_box_callback', 'ml_accordion', 'side');
}
add_action('add_meta_boxes', 'ml_accordion_meta_box');

// Callback function to render meta box
function ml_accordion_parent_meta_box_callback($post) {
    // Get all accordions
    $args = array(
        'post_type'         => 'ml_accordion',
        'posts_per_page'    => -1,
        'orderby'           => 'title',
        'order'             => 'ASC',
    );
    $accordions = get_posts($args);

    // Display select box
    ?>
    <label for="ml_accordion_parent"><?php _e('Select Parent Accordion:', 'textdomain'); ?></label>
    <select name="ml_accordion_parent" id="ml_accordion_parent">
        <option value=""><?php _e('None', 'textdomain'); ?></option>
        <?php foreach ($accordions as $accordion) : ?>
            <option value="<?php echo esc_attr($accordion->ID); ?>" <?php selected(get_post_meta($post->ID, 'ml_accordion_parent', true), $accordion->ID); ?>><?php echo esc_html($accordion->post_title); ?></option>
        <?php endforeach; ?>
    </select>
    <?php
}

// Save meta box data
function ml_accordion_save_meta($post_id) {
    if (array_key_exists('ml_accordion_parent', $_POST)) {
        update_post_meta($post_id, 'ml_accordion_parent', sanitize_text_field($_POST['ml_accordion_parent']));
    }
}
add_action('save_post', 'ml_accordion_save_meta');

wp-content\plugins\wp-multilevel-accordion\includes\post-type.php

<?php
// Exit if accessed directly.
if (!defined('ABSPATH')) {
    exit;
}

// Register custom post type for accordions
function ml_accordion_register_post_type() {
    $labels = array(
        'name'               => __('My Accordions', 'textdomain'),
        'singular_name'      => __('Accordion', 'textdomain'),
        'menu_name'      => __('Accordion', 'textdomain'),
        'add_new'      => __('Add New Accordion', 'textdomain'),
        'add_new_item'      => __('Add New Accordion', 'textdomain'),
    );
    $args = array(
        'labels'             => $labels,
        'public'             => true,
        'has_archive'        => true,
        'supports'           => array('title', 'editor'),
        'menu_icon'          => 'dashicons-list-view', // Change as needed
    );
    register_post_type('ml_accordion', $args);
}
add_action('init', 'ml_accordion_register_post_type');

wp-content\plugins\wp-multilevel-accordion\includes\shortcode.php

<?php
// Exit if accessed directly.
if (!defined('ABSPATH')) {
    exit;
}

// Shortcode to display multi-level accordion
function ml_accordion_shortcode($atts) {

    $atts = shortcode_atts(array(
        'parent' => 0, // Default parent ID
    ), $atts);

    // Query child accordions
    $args = array(
        'post_type'         => 'ml_accordion',
        'posts_per_page'    => -1,
        'orderby'           => 'menu_order',
        'order'             => 'ASC',
    );
    $child_accordions = get_posts($args);
    
    // Output accordion HTML
    ob_start();
    if ($child_accordions) :
        ?>

        <ul class="accordion" id="accordion">
            <?php foreach ($child_accordions as $accordion) : ?>

                <div class="card">
                    <div class="card-header" id="headingTwo<?= $accordion->ID ?>">
                        <h5 class="mb-0">
                            <button class="btn btn-link collapsed" data-toggle="collapse" data-target="#collapseTwo<?= $accordion->ID ?>" aria-expanded="false" aria-controls="collapseTwo<?= $accordion->ID ?>">
                            <?php echo esc_html($accordion->post_title); ?>
                            </button>
                        </h5>
                    </div>
                    <div id="collapseTwo<?= $accordion->ID ?>" class="collapse" aria-labelledby="headingTwo<?= $accordion->ID ?>" data-parent="#accordion">
                        <div class="card-body">
                            <?php echo apply_filters('the_content', $accordion->post_content); ?>
                            
                        </div>
                    </div>
                </div>

                
            <?php endforeach; ?>
        </ul>
        <?php
    endif;
    return ob_get_clean();
}

add_shortcode('ml_accordion', 'ml_accordion_shortcode');

Insert bootstrap css and js file in wp-content\plugins\wp-multilevel-accordion directory and you can download it from below links.

https://cdn.jsdelivr.net/npm/bootstrap@4.3.1/dist/js/bootstrap.min.js

https://cdn.jsdelivr.net/npm/bootstrap@4.3.1/dist/css/bootstrap.min.css