The principles of aerodynamics and fluid dynamics are what you would call "well understood."
The ambiguity is around what so-called "lift" is, which can be a nebulous concept. For example, if you drop a piece of paper it will drift slowly to the ground, essentially a form of gliding; this same air resistance is the basic force keeping a plane aloft. Would you consider this "lift"? Once you get into these arguments about semantics, things get vague.
To prove my point, in 2020 Scientific American actually published an article entitled "No One Can Explain Why Planes Stay in the Air".
Just as one example of the craziness, the FAA test, the same one you are taking, requires you to know the "four forces of flight" in which the so-called "lift" is the force that keeps the aircraft aloft. The only problem is that you can compute lift by equations that are in every book on aerodynamics and if you actually do this (like I did) you will find that the force generated is nowhere near enough to keep a plane in the sky. If "lift" were the force keeping a plane up, it would fall like a rock, so the FAA guidelines are simply completely wrong. It's just a huge semantic hairball that is not going to go away anytime soon.
The worst part of it is that EVERY pilot (or wannabe pilot) I have ever known thinks they know exactly what "lift" is and, even worse, their beliefs generally fall into one of 5 or 6 different categories with contradictory principles. This leads to huge arguments whenever the subject comes up. After 15 years of this, I just try to stay out of it, other than to tell the beginners not to make the same mistake (like I am telling you now).