What does the site manager do if not coordinating the trades (which doubtlessly includes the scheduling)?
In this case, he will be managing the construction on site.
No idea: I didn’t hire him.
This is a special arrangement here. Anyone who agrees to it will have to come to an agreement beforehand.
In my opinion, a construction supervisor (BU) instead of a general contractor (GU) would be sufficient to ensure that everyone can fulfill their role properly.
Maybe one should ask the architect what he has in mind. Includes all trades and coordinates them well, for example. :)