If you want to send transnational emails for another cc mail to the customer, then follow below steps.

Step 1: Create a registration file like Magemonkey/EmailCC/registration.php

<?php
MagentoFrameworkComponentComponentRegistrar::register(
    MagentoFrameworkComponentComponentRegistrar::MODULE,
    'Magemonkey_EmailCC',
    __DIR__
);

Step 2: Create di file like Magemonkey/EmailCC/etc/di.xml

<?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="MagentoFrameworkMailTemplateTransportBuilder">
        <plugin disabled="false" name="Magemonkey_EmailCC_Plugin_Magento_Framework_Mail_Template_TransportBuilder" sortOrder="10" type="MagemonkeyEmailCCPluginMagentoFrameworkMailTemplateTransportBuilder"/>
    </type>
</config>

Step 3: Create module file like Magemonkey/EmailCC/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="Magemonkey_EmailCC" setup_version="1.1.0">
        <sequence>
            <module name="Magento_Customer"/>
            <module name="Magento_Framework"/>
        </sequence>
    </module>
</config>

Step 4: Create extension attributes file like Magemonkey/EmailCC/etc/extension_attributes.xml

<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Api/etc/extension_attributes.xsd">
    <extension_attributes for="MagentoCustomerApiDataCustomerInterface">
        <attribute code="mm_emailcc" type="string"/>
    </extension_attributes>
</config>

Step 5: Create plugin file like Magemonkey/EmailCC/Plugin/Magento/Framework/Mail/Template/TransportBuilder.php

<?php

namespace MagemonkeyEmailCCPluginMagentoFrameworkMailTemplate;

/**
 * Plugin to add customer email cc
 */
class TransportBuilder
{
    /**
     * @var MagentoCustomerApiCustomerRepositoryInterface
     */
    protected $customerRepositoryInterface;

    /**
     * @var MagentoCustomerModelSession
     */
    protected $customerSession;

    /**
     * @var PsrLogLoggerInterface
     */
    protected $logger;

    public function __construct(
        MagentoCustomerApiCustomerRepositoryInterface $customerRepositoryInterface,
        MagentoCustomerModelSession $customerSession,
        PsrLogLoggerInterface $logger
    ) {
        $this->customerRepositoryInterface = $customerRepositoryInterface;
        $this->customerSession = $customerSession;
        $this->logger = $logger;
    }

    public function beforeGetTransport(
        MagentoFrameworkMailTemplateTransportBuilder $subject
    ) {
        try {
            $ccEmailAddresses = $this->getEmailCopyTo();
            if (!empty($ccEmailAddresses)) {
                foreach ($ccEmailAddresses as $ccEmailAddress) {
                    $subject->addCc(trim($ccEmailAddress));
                    $this->logger->debug((string) __('Added customer CC: %1', trim($ccEmailAddress)));
                }
            }
        } catch (Exception $e) {
            $this->logger->error((string) __('Failure to add customer CC: %1', $e->getMessage()));
        }
        return [];
    }

    /**
     * Get customer id from session
     */
    public function getCustomerIdFromSession()
    {
        if ($customer = $this->customerSession->getCustomer()) {
            return $customer->getId();
        }
        return null;
    }

    /**
     * Return email copy_to list
     * @return array|bool
     */
    public function getEmailCopyTo()
    {
        $customerId = $this->getCustomerIdFromSession();
        if (!$customerId) {
            return false;
        }

        $customer = $this->getCustomerById($customerId);
        if (!$customer) {
            return false;
        }

        $emailCc = $customer->getCustomAttribute('mm_emailcc');
        $customerEmailCC = $emailCc ? $emailCc->getValue() : null;

        if (!empty($customerEmailCC)) {
            return explode(',', trim($customerEmailCC));
        }

        return false;
    }

    /**
     * Get customer by Id.
     * @param int $customerId
     * @return MagentoCustomerModelDataCustomer
     */
    public function getCustomerById($customerId)
    {
        try {
            return $this->customerRepositoryInterface->getById($customerId);
        } catch (Exception $e) {
            $this->logger->critical($e);
            return false;
        }
    }
}

Step 6: Create a setup file like Magemonkey/EmailCC/Setup/Patch/Data/AddEmailCcCustomerAttribute.php

<?php

declare(strict_types=1);

namespace MagemonkeyEmailCCSetupPatchData;

use MagentoCustomerModelCustomer;
use MagentoCustomerSetupCustomerSetup;
use MagentoCustomerSetupCustomerSetupFactory;
use MagentoEavModelEntityAttributeSet;
use MagentoEavModelEntityAttributeSetFactory;
use MagentoFrameworkSetupModuleDataSetupInterface;
use MagentoFrameworkSetupPatchDataPatchInterface;
use MagentoFrameworkSetupPatchPatchRevertableInterface;

class AddEmailCcCustomerAttribute implements DataPatchInterface, PatchRevertableInterface
{

    /**
     * @var ModuleDataSetupInterface
     */
    private $moduleDataSetup;
    /**
     * @var CustomerSetup
     */
    private $customerSetupFactory;
    /**
     * @var SetFactory
     */
    private $attributeSetFactory;

    /**
     * Constructor
     *
     * @param ModuleDataSetupInterface $moduleDataSetup
     * @param CustomerSetupFactory $customerSetupFactory
     * @param SetFactory $attributeSetFactory
     */
    public function __construct(
        ModuleDataSetupInterface $moduleDataSetup,
        CustomerSetupFactory $customerSetupFactory,
        SetFactory $attributeSetFactory
    ) {
        $this->moduleDataSetup = $moduleDataSetup;
        $this->customerSetupFactory = $customerSetupFactory;
        $this->attributeSetFactory = $attributeSetFactory;
    }

    /**
     * {@inheritdoc}
     */
    public function apply()
    {
        $this->moduleDataSetup->getConnection()->startSetup();
        /** @var CustomerSetup $customerSetup */
        $customerSetup = $this->customerSetupFactory->create(['setup' => $this->moduleDataSetup]);
        $customerEntity = $customerSetup->getEavConfig()->getEntityType(Customer::ENTITY);
        $attributeSetId = $customerEntity->getDefaultAttributeSetId();
        
        /** @var $attributeSet Set */
        $attributeSet = $this->attributeSetFactory->create();
        $attributeGroupId = $attributeSet->getDefaultGroupId($attributeSetId);
        
        $customerSetup->addAttribute(
            Customer::ENTITY,
            'mm_emailcc',
            [
                'type' => 'varchar',
                'label' => 'Email CC',
                'input' => 'text',
                'source' => '',
                'required' => false,
                'visible' => true,
                'position' => 500,
                'system' => false,
                'backend' => '',
                'is_used_in_grid' => true,
                'is_visible_in_grid' => true,
                'is_filterable_in_grid' => true,
                'is_searchable_in_grid' => false,
                'note' => __("Comma separated"),
            ]
        );
        
        $attribute = $customerSetup->getEavConfig()->getAttribute(Customer::ENTITY, 'mm_emailcc');
        $attribute->addData([
            'used_in_forms' => [
                'adminhtml_customer',
                'adminhtml_checkout',
                'customer_account_create',
                'customer_account_edit'
            ]
        ]);
        $attribute->addData([
            'attribute_set_id' => $attributeSetId,
            'attribute_group_id' => $attributeGroupId
        
        ]);
        $attribute->save();

        $this->moduleDataSetup->getConnection()->endSetup();
    }

    public function revert()
    {
        $this->moduleDataSetup->getConnection()->startSetup();
        /** @var CustomerSetup $customerSetup */
        $customerSetup = $this->customerSetupFactory->create(['setup' => $this->moduleDataSetup]);
        $customerSetup->removeAttribute(Customer::ENTITY, 'mm_emailcc');
        $this->moduleDataSetup->getConnection()->endSetup();
    }

    /**
     * {@inheritdoc}
     */
    public function getAliases()
    {
        return [];
    }

    /**
     * {@inheritdoc}
     */
    public static function getDependencies()
    {
        return [];
    }
}

Step 7: Create layout file like Magemonkey/EmailCC/view/frontend/layout/customer_account_edit.xml

<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
    <update handle="customer_account"/>
    <body>
        <referenceContainer name="content">
            <block class="MagentoCustomerBlockFormEdit" name="customer_edit" template="Magento_Customer::form/edit.phtml" cacheable="false">
                <container name="form.additional.info" as="form_additional_info">
                    <block class="MagentoCustomerBlockFormEdit" as="customer_edit_cc" name="customer.edit.css" template="Magemonkey_EmailCC::form/mm_emailcc.phtml"/>
                </container>
            </block>
        </referenceContainer>
    </body>
</page>

Step 8 : Create template file like Magemonkey/EmailCC/view/frontend/templates/form/mm_emailcc.phtml

<?php
$emailCc = $block->getCustomer()->getCustomAttribute('mm_emailcc');
$customerEmailCC = $emailCc ? $emailCc->getValue() : null;
?>
<fieldset class="fieldset create account" >
    <legend class="legend">
        <span><?=$block->escapeHtmlAttr(__('Email Copy To'))?></span>
    </legend>
    <br>
    <div class="field mm_emailcc">
        <label for="mm_emailcc" class="label">
            <span><?=$block->escapeHtmlAttr(__('Email Address (Comma separated)'));?></span>
        </label>
        <div class="control">
            <input id="mm_emailcc" 
                   class="input-text" 
                   name="mm_emailcc" 
                   value="<?=$block->escapeHtmlAttr($customerEmailCC);?>" 
                   title="<?=$block->escapeHtmlAttr(__('Email Copy To'));?>" 
                   type="text" 
                   autocomplete="off" />
        </div>
    </div>
</fieldset>

Step 9: Then after run the below commands.

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

That’s it…

Now clean cache and check  “Admin > CUSTOMERS > Customers > All Customers > Edit any customer”. Your new email address cc field option should appear here in the last.