How to access a custom field in a field twig loop?
As you are creating the twig template for a field and try to access the fields / data that is not available, you can write your custom code in preprocess function and you should use the template_preproces_field and create custom variable with desired data and use that variable in custom twig template.

In preprocess function

$variables['myVariable'] = 'My Desired Data';


In Twig use the variable which will print 'My Desired Data'

{{ myVariable }}