class-admin.php

<?php

namespace WP_Plugin_Name\Inc\Admin;

use WP_Plugin_Name\Inc\Common\CommonClass;

class Admin{
  ...
    
  function foo(){
    
    $common = new CommonClass();
    $result = $common->commonFunction();
  }
}

class-common.php

<?php

namespace WP_Plugin_Name\Inc\Common;

class CommonClass {
  
  public function commonFunction(){
    
    return null;
  }
  
}