ACC works the same as normal brake pedal. it simply commands the car to slow down. The hybrid controller then determines when to use regen braking and when to use friction braking.
Most of the time, the car will use regen brake to decelerate. Friction brakes are only used in the last few mph or hard braking requests to really hold the car.
With my Prius, I had over 125K miles on the original brakes. They still had 50% life left after that. It rarely used the friction brake to slow the car down since it doesn't need to. that's why this car gets good MPG. it's recovering otherwise wasted energy. Love it.
A quick fun fact, this weekend, i was driving down from skyline blvd (in the bay area) going on Page Mill back to Los Altos. I was able to recover 12% battery, assuming 14khw usable battery capacity, i recovered 1.67kwh. To put into perspective how much energy that is, an iPhone 7 holds about 7wh of energy. I recovered enough energy to charge the iPhone 7 over 238 times!