How to automatically populate a field with a value part of another field
You probably need a custom module for this. 

Add  MODULENAME directory inside your modules directory.  Inside the directroy create file MODULENAME.info.yml with contents:

name: Last word copy
description: 'gets last word from field_a and saves it to field_b'
core: 8.x
type: module
package: COMPANYNAME
dependencies:
  - node


This module implements hook_form_alter that will make field_b readonly / disabled and hook_entity_presave which will get the last word added to field_b upon save or update of a node of CONTENTTYPE

Create also MODULENAME.module file with contents:

&lt;?php
function MODULENAME_form_alter(&amp;$form, &amp;$form_state, $form_id) { 
  if(in_array($form_id,['node_CONTENTTYPE_form','node_CONTENTTYPE_edit_form'])) {
    $form['field_b']['#disabled'] = 'disabled';
  }   
} 

function MODULENAME_entity_presave(Drupal\Core\Entity\EntityInterface $entity) {  
  if($entity-&gt;bundle() == "CONTENTTYPE"){    
    $pieces = explode(' ', $entity-&gt;field_a-&gt;value);
    $last_word = array_pop($pieces);
    $entity-&gt;field_b-&gt;value = $last_word;
    //save is done automatically
  }  
}


You NEED to replace MODULENAME and CONTENTTYPE in the above code. 

If all is correct go to Extend (admin/modules) and enable the module.