Magento 2 – How to Call Custom Block in PHTML File

(1) first create a new module with a custom eav for your product content (this is just the InstallData.php content):

<?php namespace <vendor>\<module_name>\Setup;

use Magento\Eav\Setup\EavSetup;
use Magento\Eav\Setup\EavSetupFactory;
use Magento\Framework\Setup\InstallDataInterface;
use Magento\Framework\Setup\ModuleContextInterface;
use Magento\Framework\Setup\ModuleDataSetupInterface;


/**
 * @codeCoverageIgnore
 */
class InstallData implements InstallDataInterface
{
    /**
     * EAV setup factory
     *
     * @var EavSetupFactory
     */
    private $eavSetupFactory;

    /**
     * Init
     *
     * @param EavSetupFactory $eavSetupFactory
     */
    public function __construct(EavSetupFactory $eavSetupFactory)
    {
        $this->eavSetupFactory = $eavSetupFactory;
        /* assign object to class global variable for use in other class methods */
    }

    /**
     * {@inheritdoc}
     * @SuppressWarnings(PHPMD.ExcessiveMethodLength)
     */
    public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
    {
        $setup->startSetup();
        /** @var EavSetup $eavSetup */
        $eavSetup = $this->eavSetupFactory->create(['setup' => $setup]);

        /**
         * Add attributes to the eav/attribute
         */
        $eavSetup->addAttribute(
            \Magento\Catalog\Model\Product::ENTITY,
            'my_attribute',
            [
                'type' => 'text',
                'backend' => '',
                'frontend' => '',
                'label' => 'my custom content',
                'input' => 'textarea',
                'class' => '',
                'source' => '',
                'global' => 1,
                'visible' => true,
                'required' => false,
                'user_defined' => true,
                'default' => 0,
                'searchable' => false,
                'filterable' => false,
                'comparable' => false,
                'visible_on_front' => false,
                'used_in_product_listing' => true,
                'wysiwyg_enabled' => true, // if you want a wysiwyg editor
                'unique' => false,
                'apply_to' => 'simple,configurable,virtual,bundle,downloadable,grouped',
                'group' => 'General'
            ]
        );
        $setup->endSetup();
    }
}

(2) in the module extend catalog_product_view.xml to insert your attribute: [I added mine as a new product tab]

<referenceBlock name="product.info.details">
    <block class="Magento\Catalog\Block\Product\View\Description" name="{SOME_NAME}" template="product/view/attribute.phtml" group="detailed_info">
        <arguments>
            <argument name="at_call" xsi:type="string">getMyAttribute</argument>
            <argument name="at_code" xsi:type="string">my_attribute</argument>
            <argument name="css_class" xsi:type="string">{some class if you like}</argument>
            <argument name="at_label" xsi:type="string">none</argument>
            <argument name="title" translate="true" xsi:type="string">My tab label</argument>
        </arguments>
    </block>
</referenceBlock>

(3) now you can go to your magento admin and insert custom content (even html) into the attribute content like for every other attribute

use above code in phtml file:-

<?php echo $this->getLayout()->createBlock("Magento\Catalog\Block\Product\ProductList\Upsell")->setTemplate("Magento_Catalog::product/list/items.phtml")->toHtml();?>