How do trailers attached to trucks or cars also have tail lights that correspond to the vehicle pulling it, since it can be removed?

Question

Driving in a suburban, part rural town and on the interstate, I see trailers being pulled by trucks and cars that have the brake lights go off when the vehicle’s lights go off. But the trailer doesn’t come with the car or truck when people buy it new, so how can any trailer work with any vehicle that’s hauling it?

I’m a normie, by the way. I’ve never driven a truck, nor ever hauled a trailer, so if my assumption of “any trailer can work with any vehicle” is wrong, please correct me!
Thanks for your answer!

In: Engineering

Answer 1

There’s a little cable which runs between the truck and the trailer and can easily be disconnected when removing the trailer. This cable carries electricity and the brake light signal, which turns on and off the brake lights when the truck brakes.