How to add extra custom fields in to the default Customer Registration page in Magento 2

1.Create a Module in app/code(Example: Say_Hello).

Then create a registration.php in app/code/Say/Hello/registration.php

<?php
use Magento\Framework\Component\ComponentRegistrar;
ComponentRegistrar::register(
    ComponentRegistrar::MODULE,
    'Say_Hello',
    __DIR__
);

2.Create a composer.json file for your module.

{
    "name": "say/module-hello",
    "description": "Online accessories store",
    "type": "magento2-module",
    "minimum-stability": "stable",
    "require": {
        "magento/module-catalog": "^103.0.*",
        "mageworx/module-hello": "^1.5.*"
    },
    "autoload": {
        "files": [
            "registration.php"
        ],
        "psr-4": {
            "Say\\Hello\\": ""
        }
    }
}

3.Create a module.xml in app/code/Say/Hello/etc/module.xml.

<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
	<module name="Say_Hello" setup_version="1.0.0">
        <sequence>
            <module name="Magento_Customer"/>
        </sequence>
	</module>
</config>

4.Create a InstallData.php in app/code/Say/Hello/Setup/InstallData.php(under Setup folder).For creating additional customer attribute.

<?php

namespace Say\Hello\Setup;
use Magento\Customer\Setup\CustomerSetupFactory;
use Magento\Customer\Model\Customer;
use Magento\Eav\Model\Entity\Attribute\Set as AttributeSet;
use Magento\Eav\Model\Entity\Attribute\SetFactory as AttributeSetFactory;
use Magento\Framework\Setup\InstallDataInterface;
use Magento\Framework\Setup\ModuleContextInterface;
use Magento\Framework\Setup\ModuleDataSetupInterface;
class InstallData implements InstallDataInterface
{
    /**
     * @var CustomerSetupFactory
     */
    protected $customerSetupFactory;
    /**
     * @var AttributeSetFactory
     */
    private $attributeSetFactory;
    /**
     * @param CustomerSetupFactory $customerSetupFactory
     * @param AttributeSetFactory $attributeSetFactory
     */
    public function __construct(
        CustomerSetupFactory $customerSetupFactory,
        AttributeSetFactory $attributeSetFactory
    ) {
        $this->customerSetupFactory = $customerSetupFactory;
        $this->attributeSetFactory = $attributeSetFactory;
    }
    public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
    {
        /*customersetupfactory instead of eavsetupfactory */
        $customerSetup = $this->customerSetupFactory->create(['setup' => $setup]);
        $customerEntity = $customerSetup->getEavConfig()->getEntityType('customer');
        $attributeSetId = $customerEntity->getDefaultAttributeSetId();
        /** @var $attributeSet AttributeSet */
        $attributeSet = $this->attributeSetFactory->create();
        $attributeGroupId = $attributeSet->getDefaultGroupId($attributeSetId);
        /* create customer phone number attribute */
        $customerSetup->addAttribute(Customer::ENTITY,'mobile_no',	[
                'type'         => 'varchar', 
                'label'        => 'Mobile No',
                'input'        => 'text',  
                'required'     => false, 
                'visible'      => true,
                'user_defined' => true,
                'position'     => 999,
                'sort_order'  => 999,
                'system'       => 0,
                'is_used_in_grid' => 1,  
                'is_visible_in_grid' => 1,
                'is_filterable_in_grid' => 1,
                'is_searchable_in_grid' => 1,
            ]
        );
        $sampleAttribute = $customerSetup->getEavConfig()->getAttribute(Customer::ENTITY, 'mobile_no')
            ->addData(
                [
                    'attribute_set_id' => $attributeSetId,
                    'attribute_group_id' => $attributeGroupId,
                    'used_in_forms' => ['adminhtml_customer','customer_account_edit','customer_account_create'],
                ]

        $sampleAttribute->save();
    }
}

5.Create a view file additional.phtml in app/code/Say/Hello/view/frontend/templates/additional.phtml to show additional field on customer registration form.

<div class="field mobile number required">
    <label class="label" for="mobileno">
        <span><?= $block->escapeHtml(__('Mobile No')) ?></span>
    </label>
    <div class="control">
        <input type="text" name="mobile_no" id="mobile_no" value="" title="<?= $block->escapeHtmlAttr(__('Mobile No')) ?>" class="input-text" data-validate="{required:true}">
    </div>
</div>

6.Create layout file named customer_account_create.xml in app/code/Say/Hello/view/frontend/layout/customer_account_create.xml.

<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" layout="1column" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
    <body>
        <referenceContainer name="form.additional.info">
            <block class="Magento\Framework\View\Element\Template" name="form_additional_info_customer" template="Say_Hello::additional.phtml"/>
        </referenceContainer>
    </body>
</page>

7.At last run the following command.

php bin/magento setup:upgrade
php bin/magento setup:static-content:deploy -f
php bin/magento cache:flush

The mobile field will show like this.

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