In your di.xml create a preference to your own implementation

app/code/StackExchange/PayPalPreference/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"
    >
     <preference
         for="Magento\Paypal\Model\Api\Nvp"
         type="StackExchange\PayPalPreference\Model\Api\Nvp"
     />
    </config>

In your own implementation, extend the class you want to change and override only the method in question. You will have to create a constructor matching the parent constructor, it will look something like this:

app/code/StackExchange/PayPalPreference/Model/Api/Nvp.php

    <?php
    declare(strict_types=1);
    
    namespace StackExchange\PayPalPreference\Model\Api;
    
    use Magento\Payment\Model\Method\Logger;
    
    class Nvp extends \Magento\Paypal\Model\Api\Nvp
    {
        public function __construct(
            \Magento\Customer\Helper\Address $customerAddress,
            \Psr\Log\LoggerInterface $logger,
            \Magento\Payment\Model\Method\Logger $customLogger,
            \Magento\Framework\Locale\ResolverInterface $localeResolver,
            \Magento\Directory\Model\RegionFactory $regionFactory,
            \Magento\Directory\Model\CountryFactory $countryFactory,
            \Magento\Paypal\Model\Api\ProcessableExceptionFactory $processableExceptionFactory,
            \Magento\Framework\Exception\LocalizedExceptionFactory $frameworkExceptionFactory,
            \Magento\Framework\HTTP\Adapter\CurlFactory $curlFactory,
            array $data = []
        ) {
            parent::__construct($customerAddress, $logger, $customLogger, $localeResolver, $regionFactory, $countryFactory, $processableExceptionFactory, $frameworkExceptionFactory, $curlFactory, $data);
        }
    
        protected function _applyStreetAndRegionWorkarounds(DataObject $address)
        {
            if ($address->getData('street2') !== null) {
                $address->setStreet(implode("\n", [$address->getData('street'), $address->getData('street2')]));
                $address->unsetData('street2');
            }
            if ($address->getCountryId() && $address->getRegion()) {
                $regions = $this->_countryFactory->create()
                    ->loadByCode($address->getCountryId())
                    ->getRegionCollection()
                    ->addRegionCodeOrNameFilter($address->getRegion())
                    ->setPageSize(1);
                if ($regions->count()) {
                    $regionItems = $regions->getItems();
                    $region = array_shift($regionItems);
                    $address->setRegionId($region->getId());
                    $address->setExportedKeys(array_merge($address->getExportedKeys(), ['region_id']));
                }
            }
        }
    }


You can change the app/code/StackExchange/PayPalPreference/Model/Api/Nvp.php like this and use storeManager to fetch website id to decide whether to run the code or not for particular websites:

    <?php
    declare(strict_types=1);
    
    namespace StackExchange\PayPalPreference\Model\Api;
    
    use Magento\Store\Model\StoreManagerInterface;
    
    class Nvp extends \Magento\Paypal\Model\Api\Nvp
    {
        /**
         * @var StoreManagerInterface
         */
        private StoreManagerInterface $storeManager;
    
        public function __construct(
            \Magento\Customer\Helper\Address $customerAddress,
            \Psr\Log\LoggerInterface $logger,
            \Magento\Payment\Model\Method\Logger $customLogger,
            \Magento\Framework\Locale\ResolverInterface $localeResolver,
            \Magento\Directory\Model\RegionFactory $regionFactory,
            \Magento\Directory\Model\CountryFactory $countryFactory,
            \Magento\Paypal\Model\Api\ProcessableExceptionFactory $processableExceptionFactory,
            \Magento\Framework\Exception\LocalizedExceptionFactory $frameworkExceptionFactory,
            \Magento\Framework\HTTP\Adapter\CurlFactory $curlFactory,
            StoreManagerInterface $storeManager,
            array $data = []
        ) {
            parent::__construct($customerAddress, $logger, $customLogger, $localeResolver, $regionFactory, $countryFactory, $processableExceptionFactory, $frameworkExceptionFactory, $curlFactory, $data);
    
            $this->storeManager = $storeManager;
        }
    
        protected function _applyStreetAndRegionWorkarounds(DataObject $address)
        {
            // e.g. if you do not want to run the code on website id = 2
            if ($this->storeManager->getWebsite()->getId() == 2) {
                parent::_applyStreetAndRegionWorkarounds($address);
                return;
            }
    
            if ($address->getData('street2') !== null) {
                $address->setStreet(implode("\n", [$address->getData('street'), $address->getData('street2')]));
                $address->unsetData('street2');
            }
            if ($address->getCountryId() && $address->getRegion()) {
                $regions = $this->_countryFactory->create()
                    ->loadByCode($address->getCountryId())
                    ->getRegionCollection()
                    ->addRegionCodeOrNameFilter($address->getRegion())
                    ->setPageSize(1);
                if ($regions->count()) {
                    $regionItems = $regions->getItems();
                    $region = array_shift($regionItems);
                    $address->setRegionId($region->getId());
                    $address->setExportedKeys(array_merge($address->getExportedKeys(), ['region_id']));
                }
            }
        }
    }