... There is a controler in cab that has a hard brakeline coming from it.
That hard line to controler comes from the Master Cyl there should be a T on the MC someplace that has that line to brake controler.
Then a wire goes to rear of truck from controller to actuate magnetic brakes on trailers.
Some controllers only have capability to control one axle some multiple. A check on the brand of controllers web site should provide that information. Always use the correct controller for your application,
Also NOT all brake controllers are hydraulically actuated, some have a sensor for G force and are electronic in nature or have a pendulum. Mounting the non hydraulic ones may require a certain way of being mounted for correct operation and referring to the instructions is best. DON"T just throw it in expecting it to work.
Hope this helps!