popolo_custom.module.php

<?php
function popolo_custom_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id){
    
  /**
   * Apply the form_alter to a specific form #id
   * the form #id can be found through inspecting the markup
   */
  if($form['#id'] == 'views-exposed-form-filter-articles-page-1') {

  /**
   * Include a js and css, which was defined in popolo_custom.libraries.yml
   */
  $form['#attached']['library'][] = 'popolo_custom/popolo_custom.enable';	
  $form['#attached']['library'][] = 'popolo_custom/popolo_custom.forms';

	/**
	 * Extract the options from the Views Exposed Filter <select>-list 
	 */
  $links = $form['field_tags_target_id']['#options'];

  /**
   * Iterate over the options ($links) to build an array ($pop_array) of links
   */
  $i = 0; // Initiate counter/index
  $pop_array = array();
	foreach ($links as $tid => $term_name) {
		if ($tid == 'All') {
			$pop_array[$i]['#markup'] = '<span class="filter-tab"><a href="" class="active" id="' .$tid . '">' . $term_name . '</a></span>';
		}
		else {
			$pop_array[$i]['#markup'] = '<span class="filter-tab"><a href="" id="' .$tid . '">' . $term_name . '</a></span>';
		}
		$i++; // Increase counter/index
	} 

	/**
	 * Create the item-list the form should render
	 */
		$form['links'] = [
			'#theme' => 'item_list',
		  '#items' => $pop_array,
		  '#attributes' => array('class' => array('pop-list')),
		]; 
	}
  
  else {
  	//nothing...
  } 
}