a delay timer circuit on the control board. They only use high limit swit ches for safety. If the fan stays on until the power is shutoff then I'd s uspect the fan relay on the control board is getting stuck.
Agree. It depends on the vintage of the furnace. IDK when they went to the timer based approach, but it's been a long time. Finding an install manual or following the circuit diagram that should be on the unit itself should show how this one is done.