Pre process a template that uses layout display
It appears quite easy to achieve what I want:
I needed to create a conditional field functionality. I know there is a module for that but at this time the module is not stable and not performing well with layout builder.
As indicated by @CG Monroe the last called template is field.html.twig.
If this is a user field the fully loaded user is available in:
$variables['elements']['#object']

So I could access all the user fields I needed to manipulate the field output in the preprocess function
function my_module_preprocess_field(&amp;$variables) {
  $field = $variables['elements']['#object']-&gt;field_some_other_field-&gt;value;
  // now I can manipulate the output of the field being viewed
  // the manipulation is needed because of the value of &quot;some-other-field&quot;
  $variables['items'][0]['content']['#context']['value'] = ' ***  ****';
  $variables['items'][0]['attributes']-&gt;addClass('my-module-field-manipulated');
}