retrieve parameters from url and build dynamic queries

Is this for CI3 or 4?
Look at the tutorial for a simple example:
For CI4: http://codeigniter.com/user_guide/tutori...ction.html
For CI3: http://codeigniter.com/userguide3/tutori...ction.html

Don't get the id from the model, add a parameter $id_regista to your controller. Something like that:

PHP Code:

public function regista ($id_regista = NULL)
{
    // Then call your model and pass it $id_regista as a parameter
}