Access WP custom REST Fields in custom route - php

Question

I have created custom fields to display in WordPress rest api. I want the same data to be fetched into a custom route api. How it is possible?
That is, I have some custom rest fields in the path example.com/wp-json/wp/v2/posts/
have the following rest fields I need to show in the another route. ie at example.com/wp-json/mycustom/route/v2
How can I do it?
function list_subpages() {
$data = array();
$request = array();
$my_column = array();
global $wpdb;
$save_table = $wpdb->prefix.'saved_post';
$regid = $_REQUEST['save_key'] ;
$sql1 = "select postid FROM ".$save_table." WHERE regid='".$regid."';";
$result1 = $wpdb->get_results($sql1);
foreach ($result1 as $p)
{
$id[]=$p->postid;
}
$args = array(
'post__in' => $id,
'per_page' => $per_page,
);
$subpages = get_posts( $args );
if ( empty( $subpages ) ) {
return null;
}
foreach ($subpages as $p) {
$request[] = $data;
}
return new WP_REST_Response($request, 200);
}
add_action('rest_api_init', function () {
$namespace = 'savedpost/v2';
$base = 'user';
register_rest_route($namespace, '/' . $base, array(
'methods' => 'GET',
'callback' => 'list_subpages',
));
});