golden hour

/var/www/html/wp-content/plugins/woocommerce/templates/emails/plain

File/Folder	Size	Actions
admin-cancelled-order.php	2.33 KB
admin-failed-order.php	2.31 KB
admin-new-order.php	2.23 KB
customer-completed-order.php	2.31 KB
customer-invoice.php	2.92 KB
customer-new-account.php	2.03 KB
customer-note.php	2.49 KB
customer-on-hold-order.php	2.36 KB
customer-processing-order.php	2.4 KB
customer-refunded-order.php	2.87 KB
customer-reset-password.php	2.06 KB
email-addresses.php	1.74 KB
email-customer-details.php	1 KB
email-downloads.php	1.67 KB
email-order-details.php	1.93 KB
email-order-items.php	2.25 KB

Edit: customer-processing-order.php

<?php
/**
 * Customer processing order email
 *
 * This template can be overridden by copying it to yourtheme/woocommerce/emails/plain/customer-processing-order.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 3.7.0
 */

defined( 'ABSPATH' ) || exit;

echo "=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=\n";
echo esc_html( wp_strip_all_tags( $email_heading ) );
echo "\n=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=\n\n";

/* translators: %s: Customer first name */
echo sprintf( esc_html__( 'Hi %s,', 'woocommerce' ), esc_html( $order->get_billing_first_name() ) ) . "\n\n";
/* translators: %s: Order number */
echo sprintf( esc_html__( 'Just to let you know &mdash; we\'ve received your order #%s, and it is now being processed:', 'woocommerce' ), esc_html( $order->get_order_number() ) ) . "\n\n";

/*
 * @hooked WC_Emails::order_details() Shows the order details table.
 * @hooked WC_Structured_Data::generate_order_data() Generates structured data.
 * @hooked WC_Structured_Data::output_structured_data() Outputs structured data.
 * @since 2.5.0
 */
do_action( 'woocommerce_email_order_details', $order, $sent_to_admin, $plain_text, $email );

echo "\n----------------------------------------\n\n";

/*
 * @hooked WC_Emails::order_meta() Shows order meta data.
 */
do_action( 'woocommerce_email_order_meta', $order, $sent_to_admin, $plain_text, $email );

/*
 * @hooked WC_Emails::customer_details() Shows customer details
 * @hooked WC_Emails::email_address() Shows email address
 */
do_action( 'woocommerce_email_customer_details', $order, $sent_to_admin, $plain_text, $email );

echo "\n\n----------------------------------------\n\n";

/**
 * Show user-defined additional content - this is set in each email's settings.
 */
if ( $additional_content ) {
	echo esc_html( wp_strip_all_tags( wptexturize( $additional_content ) ) );
	echo "\n\n----------------------------------------\n\n";
}

echo wp_kses_post( apply_filters( 'woocommerce_email_footer_text', get_option( 'woocommerce_email_footer_text' ) ) );