If you want to reference an already existing CMS block, and create it through XML, here’s how you do that in XML:

<block class="Magento\Cms\Block\Block" name="my.block.name">
    <arguments>
        <argument name="block_id" xsi:type="string">cms_block_id_here</argument>
    </arguments>
</block>

Then, in the template you created that block, you can just do:

<?php echo $this->getChildHtml('my.block.name') ?>