"$this->request->getVar" in a model with CI4

I have tried to use
"$this->request->getVar" in a model.
I didn't succeed.
It only worked when I moved the construct into a controller.
Why? What's my mistake?

If you need to use Request in a model method, pass a Request instance as a parameter to the method.
Or you can pass to the model constructor.
Or you can contact through the service.
There are many ways.

PHP Code:

$model = new SomeModel();
$model->someMethod($this->request);
//
$model = new SomeModel($this->request);
//
$request = App\Config\Service::request(); 


Then I tried "$this->db->update($id, $data);" to use in a model.
Error message:
Call to undefined method CodeIgniter \ Database \ MySQLi \ Connection :: update ()
Why?
How do I introduce the method to the model?

The $db property contains the connection.
To use QueryBuilder call $this->builder()

PHP Code:

$builder = $this->builder();
$builder->update(....); 
// or
$this->builder()->update(....); 


Hi.
Where does the first part placed in?

PHP Code:

$model = new SomeModel();
$model->someMethod($this->request); 


I suppose in the controller or not.

The part:

PHP Code:

$model = new SomeModel($this->request); 


gives me an Error.
'Argument 1 passed to CodeIgniter\Model::__construct() must be an instance of CodeIgniter\Database\ConnectionInterface or null, object given'
Also placed in Controller. Ok?

$request = App\Config\Service::request();
Where have this placed in?

Thanks for help.
Ralf
P.S. I am German. I hope somebody wrote in German too.