Thanks! I’ve heard a million explanations but this is clear - so the synth is taking the composition as input from what most people would think of as the “instrument” (as in, the place where somebody is picking notes and rhythm), but the synth is the thing that controls the shape of the actual soundwaves, and ideally that waveform is fully constructed within the synth from first principles, instead of just being a set of samples that are just pitch-shifted to hit each frequency to play different notes.

And obviously, adjusting the parameters synth itself is also part of the performance and composition, just as muting a trumpet or hitting an effects pedal is part of that, even though it’s not really part of “what note do I play when”, and with far more parameters available since the sound is wholly constructed instead of just being modifications of eg. a vibrating string or brass.