It should be an Account object (inherited from a User object which should be filled), see the default module: Administration. You can look for an example implementation to the Microflow with name " NewAccount". This is a standard microflow for the MxAdmin to create an user with a password. You have to do same like filling the usernames, password etc, but according to the usecase and the role who can do that.
So if you want to make a customer for example, this customer object should be created first and than be associated to the account object (or inherited creation from account could also be possible).
When logging into the Mx app, you log in with username (User.name) and password (User.password).
After the account is retrieved, the associated userroles also known by Mendix, which redirects to the role based home page.
For getting started visit pls: https://gettingstarted.mendixcloud.com/link/home
Take a look at this learning path, it covers the basics you need to work with Mendix; https://learn.mendix.com/link/path/10