custom field can be added from site admin, following code can help to get custom field value as per course object or course id.

// $course from course record from database table

if ($course instanceof stdClass) {
            $course = new core_course_list_element($course);
        }

  

$handler = \core_customfield\handler::get_handler('core_course', 'course');
       $customdata = (array)$handler->export_instance_data_object($course->id);
       $final= [];
       foreach($course->get_custom_fields() as $field){

           if($handler->can_view($field->get_field(), $course->id)){
               if(array_key_exists($field->get_field()->get('shortname'), $customdata)){
                   $final[$field->get_field()->get('shortname')] = array(
                       'name' =>$field->get_field()->get('name'),
                       'shortname' =>$field->get_field()->get('shortname'),
                       'description' =>$field->get_field()->get('description'),
                       'type' =>$field->get_field()->get('type'),
                       'data' =>$customdata[$field->get_field()->get('shortname')]
                   );
               }

           }
        }


// return output like
// array (array("name"=>"field visible name","shortname"=>"field shortname", "description"=>"field description","type"=>"field type | text| checkbox","data"=>"field data"))