Celery workers are being ran like this:
celery -A backend worker --broker=$REDIS_URL
Flower:
celery -A backend flower --broker=$REDIS_URL
When one run another worker Flower determines it. But how? Is there information stored about workers in Redis for example?
When Flower starts, it subscribes itself to be notified of most (if not all) task and worker events ( https://docs.celeryproject.org/en/stable/userguide/monitoring.html#event-reference ). When you run a new Celery worker, the moment it connects to the broker Flower will receive a new worker-online event. - That is how it finds out there is a "new worker in town"...