I created Index.php controller for that at app/code/Vendor/Modulename/Controller/Index/ and add this below code :

<?php

namespace VendorModulenameControllerIndex;

class Index extends MagentoFrameworkAppActionAction {

    /**
     * @var MagentoFrameworkViewResultPageFactory
     */
    protected $resultPageFactory;

    /**
     * @var MagentoFrameworkHTTPHeader
     */
    protected $httpHeader;

    /**
     * @param MagentoFrameworkAppActionContext      $context
     * @param MagentoFrameworkHTTPHeader             $httpHeader
     * @param MagentoFrameworkViewResultPageFactory $resultPageFactory
     */
    public function __construct(
        MagentoFrameworkAppActionContext $context,
        MagentoFrameworkHTTPHeader $httpHeader,
        MagentoFrameworkViewResultPageFactory $resultPageFactory
    ) {
        $this->httpHeader = $httpHeader;
        $this->resultPageFactory = $resultPageFactory;
        parent::__construct($context);
    }
    /**
     * Detect Mobile view or Desktop View
     *
     * @return void
     */
    public function execute() {
        $resultPage = $this->resultPageFactory->create();
        $userAgent = $this->httpHeader->getHttpUserAgent();
        $isMobile = Zend_Http_UserAgent_Mobile::match($userAgent, $_SERVER);
        if ($isMobile) {
            $resultPage->getConfig()->getTitle()->prepend(__("Mobile View")); // Mobile view logic add here
        } else {
            $resultPage->getConfig()->getTitle()->prepend(__("Desktop View")); // Desktop view logic add here
        }
        return $resultPage;
    }

}

You can add this above code in your controller or any file. You need to inject MagentoFrameworkHTTPHeader class into your construct.