checkout – How to Add Field Notice to Checkout Fields in Magento 2

1.Create a di.xml under 'etc' directory of your any custom module

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Checkout\Block\Checkout\LayoutProcessor">
        <plugin name="rewrite-checkout-fields" type="Namespace\Module\Model\Checkout\LayoutProcessorPlugin" sortOrder="10"/>
    </type>
</config>

2.Create a file called LayoutProcessorPlugin.php file under 'Namespace/Module/Model/Checkout'.

<?php
namespace Namespace\Module\Model\Checkout;

class LayoutProcessorPlugin
{
    /**
     * @param \Magento\Checkout\Block\Checkout\LayoutProcessor $subject
     * @param array $jsLayout
     * @return array
     */

    public function afterProcess(
        \Magento\Checkout\Block\Checkout\LayoutProcessor $subject,
        array  $jsLayout
    ) {
        $jsLayout['components']['checkout']['children']['steps']['children']['shipping-step']
            ['children']['shippingAddress']['children']['shipping-address-fieldset']['children']['firstname']['notice'] = __('Please enter first name here'); 
        $jsLayout['components']['checkout']['children']['steps']['children']['shipping-step']
            ['children']['shippingAddress']['children']['shipping-address-fieldset']['children']['lastname']['notice'] = __('Please enter last name here'); 
        return $jsLayout;
    }
}

Note: here i add comment below first and last name you can do for others too.Please accept and vote if use full to you this will help others too.

Add a mixin for shipping rates validation rules:

app/code/Vendor/Module/view/frontend/requirejs-config.js

var config = {
    config: {
        mixins: {
            'Magento_Checkout/js/model/shipping-rates-validation-rules': {
                'Vendor_Module/js/checkout/model/shipping-rates-validation-rules-mixin': true
            }
        }
    }
};

app/code/Vendor/Module/view/frontend/web/js/checkout/model/shipping-rates-validation-rules-mixin.js

In mixin we will wrap the original rules and add the street:

define([
    'jquery',
    'mage/utils/wrapper'
],function ($, Wrapper) {
    "use strict";

    return function (origRules) {
        origRules.getObservableFields = Wrapper.wrap(
            origRules.getObservableFields,
            function (originalAction) {
                var fields = originalAction();
                fields.push('street');

                return fields;
            }
        );
        
        return origRules;
    };
});

This will add the street rule to each available shipping method on the frontend.