In general, the threading on shower arms and shower heads are universal, they are all the same 1/2" threading. You can pretty much install any shower arm into the threaded union that is recessed inside the wall, and you can install any shower head onto the shower arm. There are always exceptions. But just look for the 1/2" threading.

Valve trim kits? In general, they should be specifically matched to the valve body that is plumbed inside the wall. If you can identify the manufacturer of the valve and the specific valve model, that will be the best way to get a new trim kit that will fit.