CI4 where should I connect to DB?

$this->db are already initialized in Model, no need to connect again. Only if you don't want to extend the "helper" model created for you.
https://codeigniter.com/user_guide/model...l-creation

You can also use all function from the query builder directly.

You don't need to do:

Code:

$this->db->countAll();

You can do:

Code:

$this->countAll();