Add description, (footnote) text or html under a checkout field in WooCommerce

woocommerce-add-text-under-checkout-field.php

// Text field
add_filter('woocommerce_form_field_text', function ($field, $key, $args, $value) {
    if ($key === 'billing_first_name') {
        $field .= 'here text <a href="/">with link</a>';
    }

    return $field;
}, 10, 4);

// Phone field
add_filter('woocommerce_form_field_tel', function ($field, $key, $args, $value) {
    if ($key === 'billing_phone') {
        $field .= 'here text <a href="/">with link</a>';
    }
    return $field;
}, 10, 4);

// E-mail field
add_filter('woocommerce_form_field_email', function ($field, $key, $args, $value) {
    if ($key === 'billing_email') {
        $field .= 'here text <a href="/">with link</a>';
    }

    return $field;
}, 10, 4);