Not every pop-up can serve as a master/controller, KP and K3 can (KP only as controller), k70 cannot if I remember correctly, I'm not sure about Kr I think it can).
You set up slaves, controller sends a preflash, slaves repond with their metering flashes so camera can meter the exposure, and then cotroller sends actual triggering signal with exposure instructions for slaves. It's all done at the speed of light, obviously.;D
Works pretty well with extra care needed for portraits in dim light, it might be harder because preflash can make models start closing their eyes.
Edit: It actually seems that optical slave mode on Godox indeed does not support p-TTL, it's just a dummy. Well that's a bummer. :P