The airflow comes from under the car, up past the front of the engine including past the red-hot headers. This underfloor air is a mixture of hot air passing thru the radiator plus more hot air laying directly on the road as the car passes over it. The whole car bottom is like a vacuum cleaner.
Our 'sugar-scoop' deck lid is a large low pressure area and generates considerable suction that pulls all that hot air up & out of the rectangular deck lid opening, then it mixes with air going over the hood and roof and on past the rear end to form a swirl following the car. The faster you go, the lower to the decklid and the further back the airflow & swirl is. Thats also why the rear mount A/C doesn't work above about 110 mph.
There are old photos (pre-cell-phone) in magazines of high speed Panteras at Bonneville and Muroc Dry Lake, that clearly show a huge contrail of salt or dust following this pattern. I don't have any of those 50-yr-old pictures personally.