The SSO solution will verify the user so the user does not need to be authenticated in the platform anymore.
The SAML module and the Open Authentication Module show examples of how this can be implemented so I would advice to review the code in these modules to see how the user is created/ provisioned (SAML module only) and logged in.
Have a look at :
https://appstore.home.mendix.com/link/app/1414/FlowFabric-BV/Open-Authentication-Module