Layout
Add block with html content
<referenceContainer name="content">
<block class="Magento\Framework\View\Element\Text" name="my.block.name">
<arguments>
<argument translate="true" name="text" xsi:type="string">
Lorem ipsum dolor ...
</argument>
</arguments>
</block>
</referenceContainer>
Add content block by id
<referenceContainer name="content">
<block class="Magento\Cms\Block\Block" name="my.content.block">
<arguments>
<argument name="block_id" xsi:type="string">my-content-block-id</argument>
</arguments>
</block>
</referenceContainer>
Add block to page head section
<referenceBlock name="head.additional">
<block class="Magento\Framework\View\Element\Text"
name="my.block.name"
template="Vendor_Module::template.phtml" />
</referenceBlock>