Yeah, I think this sim needs a thick encyclopedia-sized book to cover all of the options/settings/features and the unique qualities and attributes of operating each aircraft included.
With any one particular aircraft through testing I was often consulting with videos on Youtube with pilots/owners flying the real thing and talking their way through various procedures, as well as searching the internet for real world pilot manuals that would provide listed speeds and engine settings for various stages of flight. I also found myself searching on forums dedicated to certain aircraft types where the forum members are owners/pilots of the real aircraft. For instance, one of the peculiars about operating the XCub I found in the sim during testing was that it was quite common to experience carb icing on most flights, and after searching a Cub Crafters owners/pilots forum I found the same to be true being reported there about the real aircraft, even being described as an "ice box" (it's due to the position of the carburetor so near to the induction).
The Asobo developers have stated that all of the GA aircraft they included in this sim were picked based on they had direct access to real examples of each of them and were able to either fly them themselves, like the Robin DR400, or get first-hand accounts from the owners/pilots. They also spoke a couple weeks back in a Q&A about how they have adjusted the performance numbers to match what the pilots who fly the aircraft actually see in their aircraft, as opposed to the POH numbers which are often a bit optimistic, especially for an older aircraft with many hours on it. The devs have also stated that at some point they would like to provide an in-game feature where every user can adjust and tune their own aircraft for better/worse performance and flight handling based on things like airframe and engine hours.