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<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
    <body>
        <referenceContainer name="content">
            <block class="<vendor><Module>BlockIndex" name="helloworld.index" template="helloworld.phtml" />
        </referenceContainer>
    </body>
</page>

<?php

namespace <vendor><Module>ControllerIndex;

use MagentoFrameworkAppActionContext;

class Index extends MagentoFrameworkAppActionAction
{
    protected $_resultPageFactory;

    public function __construct(Context $context, MagentoFrameworkViewResultPageFactory $resultPageFactory)
    {
        $this->_resultPageFactory = $resultPageFactory;
        parent::__construct($context);
    }

    public function execute()
    {
        $resultPage = $this->_resultPageFactory->create();
        return $resultPage;
    }
}

<?php
namespace <Vendor><Module>Block;

class Index extends MagentoFrameworkViewElementTemplate
{
    public function getHelloWorldTxt()
    {
        return 'Hello world!';
    }
}

<h1><?php echo $this->getHelloWorldTxt(); ?></h1>