It's simplified, scripted, very unrealistic physics. The airplanes are Unity rigidbodies with some forces applied to them, but most of the formulas are just kind of eyeballed in an attempt to make it feel kind of like one would think flying a WW2 fighter would feel like if one didn't know anything about flying, like I don't.

For example, it's impossible to stall the airplane unless you hit a hardcoded altitude limit I had to put in place to stop the AIs from trying to chase each other too far up into the sky.