Can't find Meta Box Builder Taxonomy saved

Just to specify a little bit, I'd like to be able to use the value in a get_posts (meta_query). I've searched for the value in phpMyAdmin but couldn't find it.

Thanks for your answer!

I'm trying to understand but I dont get it: why is the taxonomy select value not stored in postmeta with a combination of meta_key and meta_value (as is the post select value)? rwmb_meta('field_id') is not

Here is my problem, let's say I want to get all the custom post type with a post select value of 5 and a taxonomy select value of 6. Now, what I would normally do is this:

$args = array(
  'numberposts' => -1,
  'post_type' => 'myposttype',
  'meta_query' => array(
    'relation' => 'AND',
    array(
      'key' => 'is_posttype_id', // my post select value ID
      'value' => 5
    ), // this first part above works fine
    array(
      'key' => 'is_taxonomy_id', // my taxonomy select value ID
      'value' => 6
    )
  )
);
$results = get_posts( $args );

However, as the taxonomy select value is not stored as a post meta I can't do that. So could you show me the best way to integrate the taxonomy select value in my get_posts arguments?