Hi Christian,
With the OutSystems Platform it is mandatory to use the Users entity and the User_Login action for authentication of Users, because the license structure of the OutSystems Platform is based on number of users.
In order to login users which need to be authenticated via an external service, you can call the external service, authenticate the user there and return the username of that specific user.
When this username is returned to OutSystems, you can CreateOrUpdate the Users table with this username and use the UserId from OutSystems to login (without password)
For external databases / different entities, you can do the authentication in Outsystems by hand and then also create a copy of the user in the Users entity.
Kind regards,
Remco Dekkinga