Drupal 9 coder for a callback function

It's simple, but I don't find it myself. I have a quit complex form. There are visible, checked, required states. When a use select a field, he have acces to a new select field. This select field is build with taxonomy entity reference. When the user add values to this field, and then change his mind and uncheck the first field. The values are still stored in the form_state.
I have a hook_form_alter, where I put the logic if the field is unchecked, then the values from the other field need to be unset. This need to happen in an ajax callback.
I have all the code, just need to be adjusted. Pls if you are not an expert, spare your and my time.

if ($form_state->getValue('field_a') === 0 ) {
$form_state->unsetValue('field_b');
}