I’m pretty sure that the school dance 💃 episode has confirmed Penny’s collar to be removable. And, it makes sense that Penny, being a android, wouldn’t have nipples (she can’t produce milk), genitalia (she doesn’t have ovaries, and she’s a battle robot, not a sexbot), or buttcrack (she doesn’t need to eat, so there’s no digestive system, and thus, no need to get rid of waste).