Model files in CI4

Thanks includebeer.
Regard Models, since my primary keys are a combination of several columns (in order to avoid future overflows), I need a different way to handle the DB.
The User Guide speaks about a "Manual Model Creation" (https://codeigniter4.github.io/userguide...l-creation), but I can't find how does it work.
Is it the old Ci3's $this->db object?
Copying from there, I made my new Model file: M_Comuni_2db.php

<?php
namespace App\Models;
use CodeIgniter\Database\ConnectionInterface;
class M_Comuni_2db
{
protected $db;
public function __construct(ConnectionInterface &$db)
{
$this->db = &$db;
}
public function leggi_lingue($lingua = FALSE)
{
if ($lingua === FALSE)
{
$query = $this->db->get('AA_Tabella1');
return $query->result_object();
}
// Else:
$query = $this->db->get_where('AA_Tabella1', array('Lingua' => $lingua));
return $query->row_object();
}
}

But now how can I use it from the controller?