resource
is not simple_form specific but devise specific. It is a method defined in DeviseController
which gets the actual resource stored in the instance variable of Devise model i.e., User
model in your case.
Refer to DeviseController class code in GitHub to checkout the resource method.
As you are customizing the Devise::RegistrationsController
, when you submit the form create
action of your custom RegistrationsController would be called. If you have not overridden the create
action in your custom controller then the call would propagate to the parent class I.e., Devise::RegistrationsController#create