In Australia almost all braked boat trailers use disk brakes.
They can be applied mechanically by a cable pulling levers on the calliper.
The cable is activated by the coupler. As the vehicle comes to a stop, the trailer pushes against the vehicle compressing the coupler which activates a lever. The lever has a cable that attaches to the levers on the callipers.
The coupler looks like this.
The cable attaches to the bottom hole of the lever on the left.
Another way to activate disc brakes is to use hydraulic callipers that are similar to a motor vehicle. The hydraulic master cylinder is activated by the coupler in a similar way to the cable brakes.
Hydraulic disc brakes can also be activated by an electric/hydraulic unit that is an electric hydraulic compressor that provides pressure to activate the disc brakes when it receives a signal from an in car controller. This is the US system referred to others.