M2: Add a CMS block to your layout

Add a CMS block to your layout XML:

<referenceContainer name="containerName">
    <block class="Magento\Cms\Block\Block" name="block_name_in_layout" as="block.name.in.layout">
        <arguments>
            <argument name="block_id" xsi:type="string">block_identifier</argument>
        </arguments>
    </block>
</referenceContainer>