Hi, can someone help to understand why "password" field has modifier "?" in this scheme? I don't get why the password has to be optional and why it causes an error(P1012) when I delete "?".

Not positive, but it looks like it is because you are creating a 1-1 relation without a unique tag on the user property in the password model. So it would be possible to add two passwords that reference the same user in the password table, which would break the referential invariants in the User table.