Model files in CI4

Got it!
I must use $this->db->query($sql);
So, now my Model is:

<?php
namespace App\Models;
use CodeIgniter\Database\ConnectionInterface;

class M_Comuni_2db
{
protected $db;
public function __construct()
{
$this->db = \Config\Database::connect();
}
public function leggi_lingue($lingua = FALSE)
{
if ($lingua === FALSE)
{
$sql = "SELECT * FROM AA_Tabella1";
$query = $this->db->query($sql);
$Risultato_Obj = $query->getResultObject();
}
else
{
$sql = "SELECT * FROM AA_Tabella1 where Lingua='".$lingua."'";
$query = $this->db->query($sql);
$Risultato_Obj = $query->getRow();
}
$query->freeResult();
return $Risultato_Obj;
}
}

It works.

I'll check if the Query Builder Class can work here too. If not, I can maybe make my own class for setting up the $sql string.