The exception to what Sir Humphrey has outlined would be the statutorily appointed strata manager (try say that three times with a mouthful of muesli). The statutory manager is appointed by NCAT in situations where a block is dysfunctional to the point that it is not operating properly under the terms of the Act.
That can cover a multitude of sins, from failure to hold meetings, through neglect of common property, to out and out corruption with committee members illegally granting themselves special privileges. In the case of a statutory appointment, the Owners Corporation can cease to exist as a controlling body for the duration, although there are provisions under the law for the Tribunal to specify the role of the manager (levy setting and bill payment, for instance).
However, in most cases, the owners have ceded control of their building to a manager until such times as they can show themselves to be capable of running things properly.
Having a strata manager statutorily appointed to a seriously dysfunctional block can be one of those “be careful what you wish for” moments as they are, on the one hand, expected to do things by the book, and on the other accountable to no one, so they can hire their favoured tradies at inflated rates, if they so desire.
The point is, the owners of the building have a mountain to climb if the realised belatedly that they have made a mistake. The standard appointment of an SM is one year, usually extended for a second at their request.
Otherwise, as Sir Humphrey said, the strata manager is employed by the owners corporation and usually instructed by the committee. However, in a few buildings the power chain gets twisted out of shape and you find the manager is instructing the committee and the owners at a general meeting (where the real power lies) just go along with whatever they are told.