By default in Magento 2 does not provide the mail functionality to admin when new customers signup. It means no email will be sent to the admin when a customer registers with the front side.

If you want to get the mail sent to admin when new customer signup happens then do follow below steps:

Step 1: Create a registration file like

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

Step 2: Create event file like

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="customer_register_success">
        <observer name="sendmail_toadmin" instance="MagemonkeyMailtoAdminObserverMailtoAdmin"/>
    </event>
</config>

Step 3: Create module file like Magemonkey/MailtoAdmin/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_MailtoAdmin" setup_version="1.0.0"/>
</config>

Step 4: Create PHP file in Magemonkey/MailtoAdmin/Observer/MailtoAdmin.php

<?php
 
namespace MagemonkeyMailtoAdminCustomObserver;

use MagentoFrameworkEventObserverInterface;
 
class MailtoAdmin implements ObserverInterface
{
 
    const XML_PATH_EMAIL_RECIPIENT = 'trans_email/ident_general/email';
    protected $_transportBuilder;
    protected $inlineTranslation;
    protected $scopeConfig;
    protected $storeManager;
     protected $_escaper;
    
    public function __construct(
        MagentoFrameworkMailTemplateTransportBuilder $transportBuilder,
        MagentoFrameworkTranslateInlineStateInterface $inlineTranslation,
        MagentoFrameworkAppConfigScopeConfigInterface $scopeConfig,
        MagentoStoreModelStoreManagerInterface $storeManager,
        MagentoFrameworkEscaper $escaper
    ) {
        $this->_transportBuilder = $transportBuilder;
        $this->inlineTranslation = $inlineTranslation;
        $this->scopeConfig = $scopeConfig;
        $this->storeManager = $storeManager;
        $this->_escaper = $escaper;
    }
 
    public function execute(MagentoFrameworkEventObserver $observer)
    {
        
        
        $customer = $observer->getData('customer');
        
        $this->inlineTranslation->suspend();
        try
        {
            $error = false;
            
            $sender = [
                'name' => $this->_escaper->escapeHtml($customer->getFirstName()),
                'email' => $this->_escaper->escapeHtml($customer->getEmail()),
            ];
            $postObject = new MagentoFrameworkDataObject();
            $postObject->setData($sender);
            $storeScope = MagentoStoreModelScopeInterface::SCOPE_STORE;
            $transport =
                $this->_transportBuilder
                ->setTemplateIdentifier('1')
                ->setTemplateOptions(
                    ['area' => MagentoFrameworkAppArea::AREA_FRONTEND, 
                    'store' => MagentoStoreModelStore::DEFAULT_STORE_ID,]
                )
                ->setTemplateVars(['data' => $postObject])
                ->setFrom($sender)
                ->addTo($this->scopeConfig->getValue(self::XML_PATH_EMAIL_RECIPIENT, $storeScope))
                ->getTransport();
            $transport->sendMessage(); ;
            $this->inlineTranslation->resume();
            
            
        }
        catch (Exception $e)
        {
            MagentoFrameworkAppObjectManager::getInstance()->get('PsrLogLoggerInterface')->debug($e->getMessage());
        }
    
    }
 
}

Step 5: 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 the cache and check it by creating new customer signup. You will find the mail of new customer’s sign up.