The filed widget defines how a field or collections of fields are displayed on the form. As an example is taken ProductVariationAttributesWidget of the Commerce 2.x module.

Drupal 8: alter field widget form element

<?php

/**
 * Implements hook_field_widget_WIDGET_TYPE_form_alter().
 *
 * Place this code into YOUR_MODULE.module file replacing YOUR_MODULE placeholder
 * with your modules' name.
 */
function YOUR_MODULE_field_widget_commerce_product_variation_attributes_form_alter(&$element, FormStateInterface $form_state, $context) {
  // Make ajax responsive changes on attribute fields' properties.
  // @see \Drupal\commerce_product\Plugin\Field\FieldWidget\ProductVariationAttributesWidget->formElement()
  foreach ( $context['items']->getEntity()->getPurchasedEntity()->getAttributeValueIds() as $name => $id) {
    $label = $element['attributes'][$name]['#options'][$element['attributes'][$name]['#default_value']];
    // Prettify the attribute title.
    $element['attributes'][$name]['#title'] = t("@label is a great choice for the @title!", ['@label' => $label, '@title' => $element['attributes'][$name]['#title']]);
  }
}