In the system we use in Ottawa and in the provinces, your party is elected, and you have a party leader. Whether you're in government or opposition, the party leaders then select the individuals who will fulfill various portfolios. For example, I am in the shadow cabinet as the critic for democratic institutions and Blake is the deputy critic because our leader, Rona Ambrose, at her own absolute discretion, decided to do that. She could have done the reverse. That's true for every cabinet minister as well.
However, it sounds to me as though that is not how it works here. I gather that it is not the case. You can correct me if I'm wrong, but I'm assuming that the premier does not get chosen and then turn around and say, “and this person, that person, and that person will be become members of cabinet, and I'll put them into the following portfolios.” I'm assuming that the legislature as a whole makes those decisions.