Data is stored in metafields and is available on the customer object. For example, if your machine name is "foo" for your field, the data would be available at customer.metafields.cf_app["foo"]. cf_app is the namespace and foo is the key.
If you want to use Arigato Automation to automate something with this data, like sending the extra data to Mailchimp, or using it in an email, set the namespace to cf_app and the key of foo.
Or if you wanted to use this as a condition in Arigato Automation...