forms – Proper way to get input field value with AJAX

I want to do some calculations in node edit form vith ajax. Form fields is created via UI, so i need to drill into field array to get value(s). Maximum result what i have is replacing target field with result of #defaul_value of source fields. Nothing happens when i change source.

<?
use DrupalCoreFormFormBase;
use DrupalCoreFormFormStateInterface;
use DrupalCoreAjaxAjaxResponse;
use DrupalCoreAjaxReplaceCommand;

function module_form_alter(&$form, &$form_state, $form_id) {
    
    $form('field_source')('widget')(0)('value')('#ajax')  = array(
        'callback' => 'myAjaxCallback',
        'method' => 'replace',
        'effect' => 'fade',
        'event' => 'change',
        'wrapper' => 'calc',
    );

    $form('field_source2')('widget')(0)('value')('#ajax')  = array(
        'callback' => 'myAjaxCallback',
        'method' => 'replace',
        'effect' => 'fade',
        'event' => 'change',
        'wrapper' => 'calc',
    );

    $form('field_target')('#prefix') = '<div id="calc">';
    $form('field_target')('#suffix') = '</div>';
    $form('field_target')('#disabled') = TRUE;
    
 /* some calculations */
    $s1 = $form('field_souce')('widget')(0)('value')('#default_value'); /*  HOW TO GET PROPER SOURCE VALUES "ON FLY" WHEN EDITOR CHANGES IT? */
    $s2 = $form('field_souce2')('widget')(0)('value')('#default_value');
    $result = $s1 * $s2;

    $form('field_target')('widget')(0)('value')('#default_value') = $result;
}

function myAjaxCallback($form, FormStateInterface $form_state)  {
   return $form('field_target');
}

Calculations gives result, but only with values which already submitted, so when i change source values AJAX firing & fading, but nothing happens whith target.
What i try is get values with:

$s1 = $form_state->getValue('source'); <-- gives "0" $result
$s1 = $form('field_souce')('widget')(0)('value') <-- get error Unsupported operand types