Webform 8 - How can I make a text field value be A if a select value is 1?
You have to use JS, go to settings - CSS/JS tab, write code in CUSTOM JAVASCRIPT textarea.

Some code for reference. I got a text input (id is edit-name), a select (id is edit-gender), the id you find enter 'F12' to see. ID is automatically generated by Webform.
When I select male, the text input will appear male.

(function ($, Drupal) {'use strict';$(document).ready(function(){
  'use strict';
  var name = $("#edit-name");
  var gender = $("#edit-gender");
  var duty = $("#edit-duty-select"); 
  function ab() {if (gender.val()==='male'){name.val("male");} else {name.val("girl")}};  
  gender.change(ab);  // add a Listener to select
});  
})(jQuery, Drupal);