> how can QA work in parallel on automated tests when they don't know what to test?

They can write the test cases based on the feature requirements with some boiler plate code in those tests

e.g TestUserSignUpIsSuccesful() {

   User.SignUp(email)
   User.Login(email)

   user_record = GetUserDetailFromDB(email)

   Assert.NotEmpty(user_record)
}