Magento 2 – How to Call Static Block into a 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

Replace below code with your block code

namespace Lapisbard\General\Block;
use Magento\Customer\Model\Session;

class CustomerAccount extends \Magento\Framework\View\Element\Template {
    public function __construct(\Magento\Framework\View\Element\Template\Context $context,
        Session $customerSession
    )
    {
        parent::__construct($context);
        $this->_customerSession = $customerSession;
    }
    public function isCustomerLoggedIn() {
        return $this->_customerSession->isLoggedIn();
    }
}

Then you can use it

echo $this->getLayout()->createBlock("Lapisbard\General\Block\CustomerAccount")->setTemplate("Lapisbard_General::customer_account.phtml")->toHtml();