You are right the Demo users that can be configured in the Security section is only when running locally or in a Free App or unlicensed node.
Que: For Production, when I run the app in mendix cloud (which does not work here at the moment), is there a user management built in?
No there’s not, your database would probably be a fresh instance and you would have to configure the Users, you will have MxAdmin user by default for you to login and add App Users.
Que: When I run locally… I found in “App Store” the “Administration” module, so do I have to link a page of Administration on my home page?
Yes exactly, create a Copy of the Administration module like Administration_MyApp and you can use the resources from within this module like adding the Account_Overview page to your Navigation Menu or creating a Snippet call as per your requirement.
Que: And secondly, is the system domain model extendable for additional properties where I store external username/password (which can be used for REST call)?
Technically the System module is extendable only by Generalization like System.User was generalized to Administration.Account, you should be using the Administration module more rightly a custom version of the Administration module
Hope this helps a bit and good luck.
thanx for your answers.
I can extend the class MyAdministration.Account (derived from System.User) with my own attributes
In my microflow I can access MyAdministration.Account when I choose “From database”.
EDIT: I used the following XPath constraint: [ id = $currentUser/id ] ,
which delivers the right Account/User object.