How can I embed the content of a field into the content of another field?
You can combine two fields in hook_ENTITY_TYPE_view()

function mymodule_node_view(&amp;$build, $entity, $display, $view_mode) {

  if (isset($build['field_source'][0]['#markup']) &amp;&amp; isset($build['body']['0']['#text'])) {

    $build['body']['0']['#text'] =
      '&lt;strong&gt;(' . $build['field_source'][0]['#markup'] . ')&lt;/strong&gt; ' .
      $build['body']['0']['#text'];

    unset($build['field_source']);

  }
}


and unset the added field to avoid double content.

You can also add html tags if they are allowed in the text format.

This is an example. You have to look into the build array after you've configured the display mode in ui and adjust the array keys.