woocommerce/templates/emails/plain/email-customer-details.php

<?php
/**
 * Additional Customer Details (plain)
 *
 * This is extra customer data which can be filtered by plugins. It outputs below the order item table.
 *
 * This template can be overridden by copying it to yourtheme/woocommerce/emails/plain/email-addresses.php.
 *
 * HOWEVER, on occasion WooCommerce will need to update template files and you
 * (the theme developer) will need to copy the new files to your theme to
 * maintain compatibility. We try to do this as little as possible, but it does
 * happen. When this occurs the version of the template file will be bumped and
 * the readme will list any important changes.
 *
 * @see     https://docs.woocommerce.com/document/template-structure/
 * @package WooCommerce/Templates/Emails/Plain
 * @version 3.4.0
 */

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

echo esc_html( wc_strtoupper( __( 'Customer details', 'woocommerce' ) ) ) . "\n\n";

foreach ( $fields as $field ) {
	echo wp_kses_post( $field['label'] ) . ': ' . wp_kses_post( $field['value'] ) . "\n";
}