Postal mail server codebase; "logged_in?" method not defined in ApplicationController

I just got to know of postal a few days ago and I'm very excited about all the features it offers out of the box for free. But as a junior rails dev, I'm tempted to play around with it to see if I can add my own features. I've managed to set it up and get it working. But I have a problem. I don't seem to understand what is going on in the application controller completely. There's one method "logged_in?" which get's used a lot. But it's not defined anywhere in the codebase. And I don't seem to find any gem which provides this method for me to guess it being automatically loaded in to the code, as rails does with some other modules. Below is the code:

require "authie/session"

class ApplicationController < ActionController::Base

  protect_from_forgery with: :exception

  before_action :login_required
  before_action :set_timezone

  rescue_from Authie::Session::InactiveSession, with: :auth_session_error
  rescue_from Authie::Session::ExpiredSession, with: :auth_session_error
  rescue_from Authie::Session::BrowserMismatch, with: :auth_session_error

  private

  def login_required
    return if logged_in?

    redirect_to login_path(return_to: request.fullpath)
  end

  def admin_required
    if logged_in?
      unless current_user.admin?
        render plain: "Not permitted"
      end
    else
      redirect_to login_path(return_to: request.fullpath)
    end
  end

  def require_organization_owner
    return if organization.owner == current_user

    redirect_to organization_root_path(organization), alert: "This page can only be accessed by the organization's owner (#{organization.owner.name})"
  end

  def auth_session_error(exception)
    Rails.logger.info "AuthSessionError: #{exception.class}: #{exception.message}"
    redirect_to login_path(return_to: request.fullpath)
  end

  def page_title
    @page_title ||= ["Postal"]
  end
  helper_method :page_title

  def redirect_to_with_return_to(url, *args)
    if params[:return_to].blank? || !params[:return_to].starts_with?("/")
      redirect_to url_with_return_to(url), *args
    else
      redirect_to url_with_return_to(url), *args
    end
  end

  def set_timezone
    Time.zone = logged_in? ? current_user.time_zone : "UTC"
  end

  def append_info_to_payload(payload)
    super
    payload[:ip] = request.ip
    payload[:user] = logged_in? ? current_user.id : nil
  end

  def url_with_return_to(url)
    if params[:return_to].blank? || !params[:return_to].starts_with?("/")
      url_for(url)
    else
      params[:return_to]
    end
  end

  def redirect_to_with_json(url, flash_messages = {})
    if url.is_a?(Array) && url[0] == :return_to
      url = url_with_return_to(url[1])
    else
      url = url_for(url)
    end

    flash_messages.each do |key, value|
      flash[key] = value
    end
    respond_to do |wants|
      wants.html { redirect_to url }
      wants.json { render json: { redirect_to: url } }
    end
  end

  def render_form_errors(action_name, object)
    respond_to do |wants|
      wants.html { render action_name }
      wants.json { render json: { form_errors: object.errors.full_messages }, status: :unprocessable_entity }
    end
  end

  def flash_now(type, message, options = {})
    respond_to do |wants|
      wants.html do
        flash.now[type] = message
        if options[:render_action]
          render options[:render_action]
        end
      end
      wants.json { render json: { flash: { type => message } } }
    end
  end

  def login(user)
    if logged_in?
      auth_session.invalidate!
      reset_session
    end
    Authie::Session.start(self, user: user)
    @current_user = user
  end

end