Author Fields into a content type template
Should I use include for this use case or not ?


This is not possible, you can't mix different templates like node and field because they have different variables.


  If I have too, what I'm doing wrong ? Why the item.content is not been
  load ?


item.content is only loaded in field, not in node template.


  If I should not use it in this use case, what should I do instead ?


You should define the user field you need in preprocess, examples for the two fields already defined:

function template_preprocess_node(&amp;$variables) {
  $variables['author_name'] = drupal_render($variables['elements']['uid']);
  $variables['author_picture'] = user_view($node-&gt;getOwner(), 'compact');


A basic example to use this in your own hook:

function mytheme_preprocess_node(&amp;$variables) {
  $node = $variables['node'];
  $variables['user_field1'] = $node-&gt;getOwner()-&gt;field_field1-&gt;value;


And in twig

{{ user_field1 }}