The spec, usually Division 01 or the mechanical sections, says who makes them and in what form. Many projects now require a shared 3D model.
The space above a corridor ceiling is the classic case. Ductwork, sprinkler mains, cable tray and plumbing all want the same space.
Read the requirement before you bid. Modeling and coordination meetings take real hours. Carry that cost if the spec puts it on you.
Ask the GC who leads coordination and how many meetings to expect.