How do I display username in a custom block? Am new to Drupal so this is a very fundemental question. 

How do I display the current username ? In a custom Block? I beleive is the $user-&gt;name session variable.

Do I create a Custom Block Type?  If so how do I reference the username?

Do I edit files on the file system?  If so which files?

Do I create new files? If so where? 

In other web frameworks the templating engine allows server script tags... ex.. like 

&lt;%= username %&gt;

Ive tried putting 

$user-&gt;name  and {{app.user}} 

into the custom block editor but nope, I've learned it's a WYSISYG with no way to render  

All I want to do is display a user name in a block I can add a label too.

Any help appriciated.
You can do this in sevaral ways, one of this could be use a preprocess function. In your custom module in the .module file or your custom theme in the .theme file (recommended) write this:

/**
 * Implements hook_preprocess_HOOK().
 */
function [my_module_or_theme]_preprocess_block(&amp;$variables) {
  $variables['username'] = \Drupal::currentUser()-&gt;getUsername();
} 


and then in your template suggestions of block.twig.html print the variable {{ username }}