Tutorial: WooCommerce Checkout: Add multiple custom fields to billing area. This code adds priority placement of field. Shows the custom fields in the backend Order Details screen and in e-mails to the admin and customer. I have also adjusted the checkbox field to give a text result instead of a value of 1. Checkbox is also pre-selected.

functions.php

/* --------- Adds custom fields using filters. ------ 
The below custom fields shows various types one can use.
It is then displayed in the backend Order Details page and in admin and customer e-mails.
Checkboxes by default result only show the number 1 when clicked. I have added code so that when the a checkbox is clicked it will
show text such as On and Yes. Checkbox is also pre-selected.
Tutorial: https://easywebdesigntutorials.com/woocommerce-modifying-the-checkout-page/
*/

// Initial inspiration: https://businessbloomer.com/woocommerce-add-shipping-phone-checkout/
// My Custom Fields 
// Can be added to billing, shipping or order area. For the account page use the word account.
add_filter( 'woocommerce_checkout_fields', 'custom_fields_checkout' );
function custom_fields_checkout( $fields ) {
	
   $fields['billing']['billing_checkbox'] = array(
      'label' => 'Checkbox',
      'type'  => 'checkbox',
      'required' => true,
      'class' => array( 'form-row-wide' ),
      'priority' => 26,
   );
	
    $fields['billing']['billing_text'] = array(
      'label' => 'Text',
      'type'  => 'text',
      'required' => true,
      'class' => array( 'form-row-wide' ),
      'priority' => 27,
   );
	
   $fields['billing']['billing_textarea'] = array(
      'label' => 'Text area',
      'type'  => 'textarea',
      'required' => true,
      'class' => array( 'form-row-wide' ),
      'priority' => 28,
   );

	// Added below the order notes area.
   $fields['order']['billing_date'] = array(
      'label' => 'Date',
      'type'  => 'date',
      'required' => true,
      'class' => array( 'form-row-wide' ),
      'priority' => 29,
   );
  
	// Added below the order notes area.
    $fields['order']['billing_checkbox2'] = array(
      'label' => 'Checkbox2',
      'type'  => 'checkbox',
      'default' => 1, //This will pre-select the checkbox
      'required' => true,
      'class' => array( 'form-row-wide' ),
      'priority' => 28,     
   );

	// Added below the order notes area.
    $fields['order']['billing_dropdown'] = array(
      'label' => 'Drop Down',
      'required' => true,
      'class' => array( 'form-row-wide' ),
      'priority' => 28,
      'type'  => 'select',
      'options'  	=> array( // options for <select> or <input type="radio" />
	      ''    		=> 'Please select', // empty values means that field is not selected 
                      // 'value'=>'Name'
		            	'Option 1'	=> 'Option 1 text', 
			            'Option 2'	=> 'Option 2 text',
			            'OPtion 3'	=> 'Option 3 text'
		)
      
   );
   return $fields;	
	
}


// Save the custom checkout fields in the order meta, when checkbox has been checked
// https://stackoverflow.com/questions/12958193/show-custom-field-on-order-in-woocommerce
// NB! Use this instead: https://stackoverflow.com/questions/45905237/add-a-custom-checkbox-in-woocommerce-checkout-which-value-shows-in-admin-edit-or
// Custom fields that were added to the Order area will here be shown in the Billing area inside the WP backend WooCommerce -> Order and "Order Details" page.

add_action('woocommerce_checkout_update_order_meta','my_custom_checkout_field_update_order_meta');
function my_custom_checkout_field_update_order_meta($order_id) {
  	if ( ! empty( $_POST['billing_checkbox'] ) )
        update_post_meta( $order_id, 'billing_checkbox', $_POST['billing_checkbox'] );
	
	if ( ! empty( $_POST['billing_checkbox2'] ) )
        update_post_meta( $order_id, 'billing_checkbox2', $_POST['billing_checkbox2'] );
	
 }

	      
// Display the custom field result on the order edit page (backend) when checkbox has been checked
// https://stackoverflow.com/questions/45905237/add-a-custom-checkbox-in-woocommerce-checkout-which-value-shows-in-admin-edit-or

add_action( 'woocommerce_admin_order_data_after_billing_address', 'display_custom_field_on_order_edit_pages', 10, 1 );
function display_custom_field_on_order_edit_pages( $order ){
    $billing_checkbox = get_post_meta( $order->get_id(), 'billing_checkbox', true );
    if( $billing_checkbox == 1 )
        echo '<p><strong>Checkbox: </strong> <span style="color:red;">On</span></p>';
	
	$billing_checkbox2 = get_post_meta( $order->get_id(), 'billing_checkbox2', true );
    if( $billing_checkbox2 == 1 )
        echo '<p><strong>Checkbox 2: </strong> <span style="color:red;">Yes</span></p>';
}	      

 
// Display fields in the backend Order details screen.       
add_action( 'woocommerce_admin_order_data_after_billing_address', 'customfields_billing_checkbox_checkout_display' );
function customfields_billing_checkbox_checkout_display( $order ){
	echo '<p><b>Text:</b> '      . get_post_meta( $order->get_id(), '_billing_text', true ) . '</p>';
	echo '<p><b>Text Area:</b> '  . get_post_meta( $order->get_id(), '_billing_textarea', true ) . '</p>';
	echo '<p><b>Date:</b> '      . get_post_meta( $order->get_id(), '_billing_date', true ) . '</p>';
	echo '<p><b>Drop Down:</b> ' . get_post_meta( $order->get_id(), '_billing_dropdown', true ) . '</p>';
}
	      
// Display fields in the admin and customer e-mails.  
// https://www.tychesoftwares.com/how-to-customize-woocommerce-order-emails/    	      
add_action( 'woocommerce_email_after_order_table', 'ts_email_after_order_table', 10, 4 );
function ts_email_after_order_table( $order, $sent_to_admin, $plain_text, $email ) {
        echo '<h2>Additional information</h2>';
        echo '<p><strong>'.__('Text').':</strong> <br/>' . get_post_meta( $order->get_id(), '_billing_text', true ) . '</p>';
	echo '<p><strong>'.__('Text Area').':</strong> <br/>' . get_post_meta( $order->get_id(), '_billing_textarea', true ) . '</p>';
	echo '<p><strong>'.__('Date').':</strong> <br/>' . get_post_meta( $order->get_id(), '_billing_date', true ) . '</p>';
	echo '<p><strong>'.__('Drop Down').':</strong> <br/>' . get_post_meta( $order->get_id(), '_billing_dropdown', true ) . '</p>';
	
	$billing_checkbox = get_post_meta( $order->get_id(), 'billing_checkbox', true );
    if( $billing_checkbox == 1 )
        echo '<p><strong>Checkbox: </strong> <span style="color:red;">On</span></p>';
	
	$billing_checkbox2 = get_post_meta( $order->get_id(), 'billing_checkbox2', true );
    if( $billing_checkbox2 == 1 )
        echo '<p><strong>Checkbox 2: </strong> <span style="color:red;">Yes</span></p>';      
}