usually the fan hook is installed on the concrete ceiling / rafter 1st before installing the roof.
Then once cable and fittings are done, then the plaster ceiling would come into play.
This is why plaster ceiling are considered as finishes. because it is one of the components that comes in during the last stage of construction.

If you already have a plaster ceiling installed, then if im not mistaken, you have to punch a whole through the ceiling and drill the hook in, then you put the fans leg inside, through the ceiling.
Once that is done, you can later on cover the extra opening on the ceiling with the fans cap given.