I’d imagine that’s the case. FA are told what the carousel and they announce it to the passengers. Even FlightRadar24 has the Bagage Belt announced on some flights.
Most airports have a ramp/company frequency, they probably talk to them and the pilot hands the info to the FA. I’d assume it’s just given whichever is free.
I wonder how far ahead they decide which carousel it’ll be? If it’s decided far enough in advance, the flight attendants could find out on their crew devices/apps/however that works which carousel they’ll be assigned before they even take off.