Having just gone through the process of getting acquainted with the idea of a bike's fit, I've found that a big part of the process is just becoming aware / conscious, while riding, of how I'm made, how the bike is shaped and what's going on between my body and the bike. The best fit will depend on the type of bike and how you want to be positioned on it. I've read up quite a bit on the sites of the various fitting providers and doing so gave me the words and concepts to understand what's going on. The theory seems to take for granted that you'll maintain the same position on the bike all the time. I move around a bit on my bike to give different parts a break and have noticed that while there's really only one position that is ideally fitted, I can't maintain it,without moving, for hours on end.
Steve Hogg's site is very instructive :
https://www.stevehoggbikefitting.com/, under the "Self Help" tab.