If you’re talking adaptive cruise, yes service brakes are employed automatically as needed.
I have one of them there fancy automagical braking cars now and it’s a friggin death trap. Hate that feature. I leave it off unless I’m physically paying attention to how I think it will react. It’s not very smart and has either locked up the brakes down to a dead stop or hit the brakes pretty hard multiple times now when it “sensed” an impending collision that was not going to happen. I believe it will stop some true rear ender collisions if it acts like it does in a real impending collision, but when it’s not thinking right, it makes the driver look stupid and increases the chance of another collision.
Adaptive cruise and forward collision warning shouldn’t be used while towing imo.