I’m not sure how to accurately ask this, but I’m curious what effect a more forward vs more rearward seat position has on pedaling efficiency or muscle engagement. Is one position better than the other for a particular discipline, say, racing vs long distance? Does the hip joint’s relationship to the bottom bracket affect anything?

Peter White, who produces and sells bike equipment has a fine description of the various elements of bike fit. Reading his description highlights how much of a compromise a bike fit can be to find an optimum between racing or cruising around town or setting a bike up for a world tour. https://www.peterwhitecycles.com/fitting.php

KOPS is always a good starting point. Saddle more forward opens the hip angle, which is good for aero position, but it weights the hands more. It increases quad activity. Saddle more aft takes weight off the hands, increases hamstring and glute activity. Some long distance riders prefer a slightly more aft position. Time trialists have a much more forward position. Of course changing saddle position changes reach, thus moving the saddle forward means a longer stem and vice versa. Preferred reach is what you don't change.

Riding style can also be a factor. No matter where I place my saddle, I seem to like riding on the nose. I imagine a coach might get me to change, but I ride for fun and for exercise, and my competition is with myself. Besides, my budget can't stretch for a coach, so ... I do what comes naturally. I suspect at least some other riders also use their own style.