It’s been said that many of the aircraft in the Base through Premium packages were developed by 3rd parties. So there’s that.
I’m pretty sure at least some of the planes, however, were developed by Asobo straight out, like the C172, which, if you go through the Dev Q&A’s, will find they instrumented themselves and tried to match the performance of it to their measurements in several flight regimes, not just to the POH numbers.