If your appointment event has more than one operator, you can auto-assign them depending on the availability of the operators added for that event.

You do that from the Operators & Services tab when editing an event. You will see a checkbox to assign operators automatically if the booked time is available in more than one of the operators in the meeting.

Go to Appointments

Click on Booking Events

Click on the event you want to auto-assign operators to and check "Assing the operators automatically.