I’m more using it in the colloquial sense of “complicated” exercises. Whether this happens all at once or via transitioning from one exercise to another, it requires more skill and coordination than working muscles in isolation. Not to be confused with noun complexes which actually has a specific meaning in the context of lifting.
There’s technically a distinction between compound lifts (squat, clean) and lifting complexes (clean + jerk; clean + push press).
I’ve really only used the latter when I was training olympic weightlifting or some sport specific skills. eg, you might do 2 cleans and then a jerk because you want to practice jerking with more fatigue. Some more info on complexes if you’re interested.
Take this 1 DB thruster + 2 DB push press complex. You’re going to get more strength gains from doing these movements separately, with appropriate weight. But say you were an American football player. This is not too different from the mechanics one would experience on the line. Theoretically, it could be beneficial to get good at having a strong, repeated push press after the fatigue of a thruster. Does this “skill” make sense for a cyclist? Probably not.
I think you’re actually hitting on a different concept here, which is whether the exercise is bilateral (squat), unilateral (single-leg leg press), ipsilateral (same side single-leg RDL), or contralateral (single-leg RDL). I’m not sure the latter two are possible in movements that are not compound in some way, because it always seem to implicate the trunk? I’d have to think about this more. But contralateral, ipsilateral, and (some) unilateral exercises tend to train stability, which is often beneficial. However, they are rarely going to be your best bets for strength/hypertrophy stimulus, unless the target group is actually core. They are often accessory exercises that compliment primary exercises.
For example, single-leg RDLs are typically loaded contralaterally, which really hits the glute minimus as a stabilizer. Your stability and balance will fail on these before you reach a weight that will provide optimal stimulation for hams and glute max. But that’s fine. Minimus is getting a workout and it’s getting trained within the context of the deadlift movement. Single-leg RDLs are often an accessory lift to deadlift. Your deadlift will benefit from the stability training, along with your general athleticism (you’d be hard pressed to find a scenario where hip stability is bad).