This is established in the Pundit application policy in app/policies/application_policy.rb. Basically, all index views make a call to the policy_scope helper, i.e. below is the index action of the Donors controller:

def index
  @records = policy_scope(Donor).page params[:page]
end

If you read the Pundit documentation, you'll discover that this helper is a shortcut for calling the Scope#resolve method, that lives in whichever policy class is registered with the model. In Pulsar LIMS, all models are registered with the default ApplicationPolicy class that the Pundit generator gives you, as all models set the following class method:

def self.policy_class
  ApplicationPolicy
end

Thus, setting the default sort order in ApplicationPolicy::Scope#resolve is the simplest and most straightforward way, and doesn't require setting it explicitly in each index action.