<?php 

namespace Vendor\MagentoCustomer\Observer;

use Magento\Framework\Event\ObserverInterface;

class CustomerLogin implements ObserverInterface {
   /**     
     * @param \Magento\Checkout\Model\Session
     */
    protected $checkoutSession;
/**
 * @var \Magento\Quote\Api\CartRepositoryInterface
 */
    protected $quoteRepository;


 public function __construct(
        \Magento\Checkout\Model\Session $checkoutSession,
        \Magento\Quote\Api\CartRepositoryInterface $quoteRepository
    ) {
        $this->checkoutSession = $checkoutSession;
        $this->quoteRepository = $quoteRepository;
    }
    
    public function execute(\Magento\Framework\Event\Observer $observer)
    {        
        $customer = $observer->getEvent()->getCustomer();       
        $customerId = $customer->getId();
        if($customerId){            
        //method 1
            $quote= $this->checkoutSession->getQuote()->loadByCustomer($customerId); 
            $cartItems=$quote->getAllItems();
            echo count(cartItems);
        //method 2
          $customerQuote = $this->quoteRepository->getForCustomer($customerId);
        $cartItems2=$customerQuote->getAllItems();
        echo count($cartItems2);die;

       }
    }
}

$this->checkoutSession->getQuote()->loadByCustomer($customerId);

$this->quoteRepository->getForCustomer($customerId);

these line will help you to fetch current customer cart items

<?php 

namespace Vendor\MagentoCustomer\Observer;

use Magento\Framework\Event\ObserverInterface;

class CustomerLogin implements ObserverInterface {
   /**     
     * @param \Magento\Checkout\Model\Session
     */
    protected $checkoutSession;
/**
 * @var \Magento\Quote\Api\CartRepositoryInterface
 */
    protected $quoteRepository;


 public function __construct(
        \Magento\Checkout\Model\Session $checkoutSession,
        \Magento\Quote\Api\CartRepositoryInterface $quoteRepository
    ) {
        $this->checkoutSession = $checkoutSession;
        $this->quoteRepository = $quoteRepository;
    }
    
    public function execute(\Magento\Framework\Event\Observer $observer)
    {        
        $customer = $observer->getEvent()->getCustomer();       
        $customerId = $customer->getId();
        if($customerId){            
        //method 1
            $quote= $this->checkoutSession->getQuote()->loadByCustomer($customerId); 
            $cartItems1=$quote->getAllItems();
            echo count(cartItems1);echo '<br>';
        //(OR) method 2
          $customerQuote = $this->quoteRepository->getForCustomer($customerId);
        $cartItems2=$customerQuote->getAllItems();
        echo count($cartItems2);echo '<br>';
       //(OR) method 3
        $quoteId = $this->checkoutSession->getQuote()->getId();
        $quote = $this->checkoutSession->getQuote()->load($quoteId);
        $cartItems3=$quote->getAllItems();
        echo count($cartItems3);
       }
    }
}

$this->checkoutSession->getQuote()->loadByCustomer($customerId);

OR

$this->quoteRepository->getForCustomer($customerId);

OR

$quote = $this->checkoutSession->getQuote()->load($quoteId);

All above methods can help you to fetch current customer cart items. Hope this helps.

<?php 
namespace Vendor\MagentoCustomer\Observer;

use Magento\Framework\Event\ObserverInterface;

class CustomerLogin implements ObserverInterface {
   /**     
     * @param \Magento\Checkout\Model\Session
     */
    protected $checkoutSession;
 public function __construct(
        \Magento\Checkout\Model\Session $checkoutSession
    ) {
        $this->checkoutSession = $checkoutSession;
    }
    
    public function execute(\Magento\Framework\Event\Observer $observer)
    {        
        $customer = $observer->getEvent()->getCustomer();       
        $customerId = $customer->getId();
        if($customerId){            
            $quote= $this->checkoutSession->getQuote()->loadByCustomer($customerId); 
            $cartItems=$quote->getAllItems();
            echo count(cartItems);die;
    }
}

$quote= $this->checkoutSession->getQuote()->loadByCustomer($customerId);

this line will help you to fetch current customer cart items

<?php  

namespace Vendor\MagentoCustomer\Observer;

use Magento\Framework\Event\ObserverInterface;

class CustomerLogin implements ObserverInterface {
   /**     
     * @param \Magento\Checkout\Model\Session
     */
    protected $checkoutSession;
/**
 * @var \Magento\Quote\Api\CartRepositoryInterface
 */
    protected $quoteRepository;


 public function __construct(
        \Magento\Checkout\Model\Session $checkoutSession,
        \Magento\Quote\Api\CartRepositoryInterface $quoteRepository
    ) {
        $this->checkoutSession = $checkoutSession;
        $this->quoteRepository = $quoteRepository;
    }
    
    public function execute(\Magento\Framework\Event\Observer $observer)
    {        
        $customer = $observer->getEvent()->getCustomer();       
        $customerId = $customer->getId();
        if($customerId){             
        //method 1
            $quote= $this->checkoutSession->getQuote()->loadByCustomer($customerId); 
            $cartItems=$quote->getAllItems();
            echo count(cartItems);
        //method 2
          $customerQuote = $this->quoteRepository->getForCustomer($customerId);
        $cartItems2=$customerQuote->getAllItems();
        echo count($cartItems2);die; 

       }
    }
}

$this->checkoutSession->getQuote()->loadByCustomer($customerId);

$this->quoteRepository->getForCustomer($customerId);

these line will help you to fetch current customer cart items