Magento 2 : Update layout handle from XML by condition

You can do that using Controller action

You can use below code

<?php
namespace Vendor\Module\Controller\Index;

class Index extends \Magento\Framework\App\Action\Action
{
    protected $resultPageFactory;

    public function __construct(
        \Magento\Framework\App\Action\Context $context,
        \Magento\Framework\View\Result\PageFactory $resultPageFactory
    ){
        $this->resultPageFactory = $resultPageFactory;
        return parent::__construct($context);
    }

    public function execute()
    {
        $resultPage = $this->resultPageFactory->create();

        $config = $this->getConfig('test/test/test');
        if($config){
            $resultPage->addHandle('my_custom_layout_1');
        }else{
            $resultPage->addHandle('my_custom_layout_2');
        }

        return $resultPage;
    }
}

Hope this will help you!

You could use a plugin

https://github.com/DominicWatts/MaintenancePage/blob/1a1ce757adae972f6b5435e33da11ea21d5312a0/Plugin/Frontend/Magento/Framework/View/Result/Layout.php#L30-L43

    /**
     * @param \Magento\Framework\View\Result\Layout $subject
     * @return \Magento\Framework\View\Result\Layout
     * @throws \Magento\Framework\Exception\NoSuchEntityException
     */
    public function afterAddDefaultHandle(
        \Magento\Framework\View\Result\Layout $subject
    ) {
        $storeId = $this->storeManager->getStore()->getId();
        if ($this->pageHelper->isEnabled($storeId) && !$this->pageHelper->isDevAllowed($storeId)) {
            $subject->addHandle('maintenance_page_mode');
        }
        return $subject;
    }

See how it's based on config

https://github.com/DominicWatts/MaintenancePage/blob/1a1ce757adae972f6b5435e33da11ea21d5312a0/Helper/Page.php#L67-L79

    /**
     * Maintenance page
     * @param null $storeId
     * @return mixed
     */
    public function isEnabled($storeId = null)
    {
        return $this->scopeConfig->getValue(
            self::XML_PATH_MAINTENANCE_ENABLED,
            \Magento\Store\Model\ScopeInterface::SCOPE_STORE,
            $storeId
        );
    }