whenyou open your hood you'll see a resevoir next to the engine. It sits on top of the Power Split Device mentioned by Seawolf34. It carries the coolant for the PSD.
There is no traditional transmission just the PSD. According to everything I've read online and in speaking with our Gen Service Mgr ( Prius Master Mechanic ) this coolant uses a long-lsting formulation which only needs to be changed every 100K or so. Changing it at 75K wouldn't hurt if you so desire.
One of the things to check apparently is that the system be fully drained and fully refilled. They attach clamps to the sides of the car and turn it upside down to shake out the last drop of the old coolant (
j/k ). I'd recommend that someone very versed in doing the coolant swap actually do the work.