How to Add Custom Data to WooCommerce Order

Step 1: Add Data in a Custom Session, on ‘Add to Cart’ Button Click
<?php
add_action('wp_ajax_wdm_add_user_custom_data_options', 'wdm_add_user_custom_data_options_callback');
add_action('wp_ajax_nopriv_wdm_add_user_custom_data_options', 'wdm_add_user_custom_data_options_callback');

function wdm_add_user_custom_data_options_callback()
{
      //Custom data - Sent Via AJAX post method
      $product_id = $_POST['id']; //This is product ID
      $user_custom_data_values =  $_POST['user_data']; //This is User custom value sent via AJAX
      session_start();
      $_SESSION['wdm_user_custom_data'] = $user_custom_data_values;
      die();
}
Step 2: Add Custom Data in WooCommerce Session
add_filter('woocommerce_add_cart_item_data','wdm_add_item_data',1,2);
 
if(!function_exists('wdm_add_item_data'))
{
    function wdm_add_item_data($cart_item_data,$product_id)
    {
        /*Here, We are adding item in WooCommerce session with, wdm_user_custom_data_value name*/
        global $woocommerce;
        session_start();    
        if (isset($_SESSION['wdm_user_custom_data'])) {
            $option = $_SESSION['wdm_user_custom_data'];       
            $new_value = array('wdm_user_custom_data_value' => $option);
        }
        if(empty($option))
            return $cart_item_data;
        else
        {    
            if(empty($cart_item_data))
                return $new_value;
            else
                return array_merge($cart_item_data,$new_value);
        }
        unset($_SESSION['wdm_user_custom_data']); 
        //Unset our custom session variable, as it is no longer needed.
    }
}
Step 3: Extract Custom Data from WooCommerce Session and Insert it into Cart Object
add_filter('woocommerce_get_cart_item_from_session', 'wdm_get_cart_items_from_session', 1, 3 );
if(!function_exists('wdm_get_cart_items_from_session'))
{
    function wdm_get_cart_items_from_session($item,$values,$key)
    {
        if (array_key_exists( 'wdm_user_custom_data_value', $values ) )
        {
        $item['wdm_user_custom_data_value'] = $values['wdm_user_custom_data_value'];
        }       
        return $item;
    }
}
Step 4: Display User Custom Data on Cart and Checkout page
add_filter('woocommerce_checkout_cart_item_quantity','wdm_add_user_custom_option_from_session_into_cart',1,3);  
add_filter('woocommerce_cart_item_price','wdm_add_user_custom_option_from_session_into_cart',1,3);
if(!function_exists('wdm_add_user_custom_option_from_session_into_cart'))
{
 function wdm_add_user_custom_option_from_session_into_cart($product_name, $values, $cart_item_key )
    {
        /*code to add custom data on Cart & checkout Page*/    
        if(count($values['wdm_user_custom_data_value']) > 0)
        {
            $return_string = $product_name . "</a><dl class='variation'>";
            $return_string .= "<table class='wdm_options_table' id='" . $values['product_id'] . "'>";
            $return_string .= "<tr><td>" . $values['wdm_user_custom_data_value'] . "</td></tr>";
            $return_string .= "</table></dl>"; 
            return $return_string;
        }
        else
        {
            return $product_name;
        }
    }
}

Step 5: Add Custom Data as Metadata to the Order Items
add_action('woocommerce_add_order_item_meta','wdm_add_values_to_order_item_meta',1,2);
if(!function_exists('wdm_add_values_to_order_item_meta'))
{
  function wdm_add_values_to_order_item_meta($item_id, $values)
  {
        global $woocommerce,$wpdb;
        $user_custom_values = $values['wdm_user_custom_data_value'];
        if(!empty($user_custom_values))
        {
            wc_add_order_item_meta($item_id,'wdm_user_custom_data',$user_custom_values);  
        }
  }
}
Step 6: Remove User Custom Data, if Product is Removed from Cart
add_action('woocommerce_before_cart_item_quantity_zero','wdm_remove_user_custom_data_options_from_cart',1,1);
if(!function_exists('wdm_remove_user_custom_data_options_from_cart'))
{
    function wdm_remove_user_custom_data_options_from_cart($cart_item_key)
    {
        global $woocommerce;
        // Get cart
        $cart = $woocommerce->cart->get_cart();
        // For each item in cart, if item is upsell of deleted product, delete it
        foreach( $cart as $key => $values)
        {
        if ( $values['wdm_user_custom_data_value'] == $cart_item_key )
            unset( $woocommerce->cart->cart_contents[ $key ] );
        }
    }
}
?>

Reference link: https://wisdmlabs.com/blog/add-custom-data-woocommerce-order/