Very likely in a similar way to how we did Seated Passengers in GSX:
They are “ModelAttachments” for the whole flight (so we don’t have to code anything or generate any traffic to keep them glued with the airplane for the whole flight), with a Visibility check that would make them invisible during Boarding/Deboarding, which is performed with standard Simconnect commands, it’s just a matter of synching the switch between the two models as seamless as possible.