The constructor is the ship that builds orbital stations and gates. It builds a station where it is, and a standard gate or waygate in its current system pointing at a charted system. Every build is paid for from its own hold.
A Shipyard builds it from thrusters, frames and circuits. It is a frigate, so it fits every station's dock, and it has a larger hold than a freight ship.
The hull builds only an Outpost or a Workshop, and a lander builds only surface stations. Every other station, and every gate, takes a constructor or an ark.