Code for adding check box field:

// checkbox field
add_action( 'woocommerce_after_order_notes', 'quadlayers_subscribe_checkout' );
function quadlayers_subscribe_checkout( $checkout ) {
woocommerce_form_field( 'subscriber', array(
'type' => 'checkbox',
//'required' => true,
'class' => array('custom-field form-row-wide'),
'label' => ' Subscribe to our newsletter.'
), $checkout->get_value( 'subscriber' ) );
}

Code for adding radio buttons field:

// Radio input field
add_action('woocommerce_before_order_notes', 'quadlayers_radio_checkout');
function quadlayers_radio_checkout($checkout3){
woocommerce_form_field( 'feed', array(
'type' => 'radio',
//'required' => true,
'class' => array('custom-field form-row-wide'),
'label' => ' How did you find us?.',
'options' => array(
'Google' => 'Google',
'Friend' => 'Friend',
'Facebook' => 'Facebook',
'Youtube' => 'YoutTube',
'Other' => 'Other'
)
));
}

Likewise, you can add as many fields as you want.

• Now, save the added fields to the databases. Add the following code to the script.

add_action( 'woocommerce_checkout_update_order_meta','quadlayers_save_function' );
function quadlayers_save_function( $order_id ){
if ( ! empty( $_POST['subscriber'] ) ) {
update_post_meta( $order_id, 'subscriber', sanitize_text_field( $_POST['subscriber'] ) );
}
if ( ! empty( $_POST['feed'] ) ) {
update_post_meta( $order_id, 'feed',sanitize_text_field( $_POST['feed'] ) );
}
}

• Make unnecessary fields optional by adding this code snippet.

add_filter( 'woocommerce_billing_fields', 'wc_address_field_optional');
function wc_address_field_optional( $fields ) {
$fields['billing']['billing_address_1']['required'] = false; 
return $fields; 
}

• Add content to the page.

Adding Image to the checkout page:

add_action('woocommerce_review_order_before_submit','quadlayers_checkout_content');
function quadlayers_checkout_content(){
echo <img src="https://www.heresylab.com/wp-content/uploads/2019/01/paypal-1.png"/>;
}

Adding heading to the checkout page:

add_action(woocommerce_checkout_before_customer_details
,'quadlayers_checkout_header');
function quadlayers_checkout_header(){
echo "
This is a custom Header
";
}

• You can also use shortcode here in functions.php file to add a page.

add_action('woocommerce_after_checkout_form','quadlayers_checkout_shortcode');
function quadlayers_checkout_shortcode(){
echo do_shortcode('[woocommerce_cart]'); }

• Edit the WooCommerce checkout page with custom CSS.

add_action( 'wp_enqueue_scripts', 'quadlayers_enqueue_css' );
function quadlayers_enqueue_css(){    
    wp_enqueue_style(  'checkout_style',
        get_stylesheet_directory_uri() . '/checkout-style.css'
    );
}