Untitled

<?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>
        <referenceBlock name="product_form">
            <block class="VendorDemoBlockAdminhtmlCatalogProductEditTabDemo" name="demo.tab">
                <arguments>
                    <argument name="config" xsi:type="array">
                        <item name="label" xsi:type="string" translate="true">Demo Tab</item>
                        <item name="collapsible" xsi:type="boolean">true</item>
                        <item name="opened" xsi:type="boolean">true</item>
                        <item name="sortOrder" xsi:type="string">2</item>
                        <item name="canShow" xsi:type="boolean">true</item>
                        <item name="componentType" xsi:type="string">fieldset</item>
                    </argument>
                </arguments>
            </block>
        </referenceBlock>
    </body>
</page>

<?php
namespace VendorDemoBlockAdminhtmlCatalogProductEditTab;

use MagentoBackendBlockTemplateContext;
use MagentoFrameworkRegistry;

class Demo extends MagentoFrameworkViewElementTemplate
{
    /**
     * @var string
     */
    protected $_template = 'product/edit/demo.phtml';

    /**
     * Core registry
     *
     * @var Registry
     */
    protected $_coreRegistry = null;

    public function __construct(
        Context $context,
        Registry $registry,
        array $data = []
    )
    {
        $this->_coreRegistry = $registry;
        parent::__construct($context, $data);
    }

    /**
     * Retrieve product
     *
     * @return MagentoCatalogModelProduct
     */
    public function getProduct()
    {
        return $this->_coreRegistry->registry('current_product');
    }

}