What about Mass Air Pressure

You mentioned two ways of measuring air flow, and that the aftermarket uses a principle. But you didn't say what way it actually accomplishes it? Or perhaps I'm jumping ahead

Here's my best guess at what you're asking and I hope this helps.

Mass airflow is commonly measured with a mass airflow sensor.

If a mass airflow sensor is not fitted, or not capable of measuring accurately post modifications, mass airflow can be inferred by a method sometimes called Speed Density, which uses manifold air pressure, air temperature and other data and math to infer/approximate what the mass airflow is.

In the practical tuning courses, both methods of airflow calculation, and how to tune them, are taught.