There are different types of contacts. For example, if someone is an actual family member or a fellow traveller, they would have had much closer and more prolonged exposure to the case. Those are the high-risk contacts. They will be essentially in isolation or quarantine for 14 days to protect against any further transmission. Other passengers or other contacts will be followed up on by local public health using their protocols.
Right now it is not necessarily keeping them completely isolated. It involves active monitoring by the local public health departments.