In this setting I imagine the planet has about 4.2 atmospheres of pressure at sea level and 1.17 G's, this is not necessarily fixed and if other values would allow earthlike life and human civilization, and produce more interesting effects then feel free to solve for other gravity values and atmosphere pressures, but both values are higher than on earth.
The planet in question is earthlike but larger with more gravity and a thicker atmosphere. Since a planet with a earthlike atmosphere, but higher atm's would have a higher partial pressure for oxygen and nitrogen, I think it could be reasonable for life to have evolved to deal with that so as to avoid toxicity from those elements. If the atmosphere just can't be plausibly similar to earths, but still allow earthlike life under these high pressures, assume it is different.
As for the aviation technology most things are the same, but there isn't any computing and engines can't generate any more thrust than a muscle of the same size limited by realistic earth biology, because magically grown muscles are the only power source. This severely limits how much power an aircraft can have. Certain materials are cheap and easy to to produce in this setting such as spider silk and limpet teeth like materials, and rare and heavy elements are far more common on this planet. Other than muscle power the only other form of propulsion is rockets, but the rockets have to use magically generated explosions, as thus the rockets are more like nuclear Orion rockets than traditional ones, though the explosions can be smaller down to that of a large cannon.
My question is, how would you expect aircraft and rockets to be different in this environment, with power limited to magically grown muscles (limited by biological realism, they're only grown and controlled via magic), and magical explosions, but more available rare materials, and different gravitational and atmospheric conditions?