Compositing (transparent BG) video or image sequences in Unreal

These guys have obviously managed to figure out a way to composite green-screen footage an unreal. I’m assuming they have some sort of real time feed going in from camera to the engine, but shouldn’t the same thing be possible with video that’s already been recorded?

I mean whatever these guys are doing is basically what I’m trying to do. The only difference is that my footage is cartoons and not live actors.