Drupal 8 hook_form_alter

Dear everyone,

I've been struggling for almost a week now trying to learn how to implement hook_form_alter in Drupal 8. I've got the Form API module from the examples module and I've been trying to insert hook_form_alter there but Drupal doesn't even see this function.

The routing file looks like this:

fapi_example.description:
  path: 'examples/fapi-example'
  defaults:
    _controller:  '\Drupal\fapi_example\Controller\Page::description'
    _title: 'Form API Examples'
  requirements:
    _permission: 'access content'

fapi_example.simple_form:
  path: 'examples/fapi-example/simple-form'
  defaults:
    _form:  '\Drupal\fapi_example\Form\SimpleForm'
    _title: 'Simple Form API Example'
  requirements:
    _permission: 'access content'

And I have my hook_form_alter function like this:

 function fapi_example_simple_form_form_alter(&$form, &$form_state) {
    $form['title']['#title'] = t('ANYTHING HERE!');
	kint($form);
  } 

So where exactly should I implement the above function? Inside the controller file or inside the Form file. Should it be a method of in the class that extends FomBase or where? I appreciate any help available. Thank you all in advance.