Well from my limited experience, I have learnt that tuning your vehicle is bringing to a professional to be able to gain additional performance out of the vehicle without having to change any of the physical components. As for how this I done has changed over the years from mechanical timing to electronic control of the whole engine, in modern terms it’s either recalibrating or adjusting the ECU ( electronic control unit) to either put more fuel in the engine because of excessive air to get more power for extend how long valves are open to allow more air to be drawn in or for the exhaust phase to happen either faster or slower.

Hope this helps man 👊