I dont like thinking of bits of horses or riders as bits of lego, isolated and chunked off. I think of horses and riders as watercolour paintings.
Imagine I lightly sketched a pencil outline of a horse and rider on a piece of watercolour paper. Now I take plain water, and wet the paper between the lines. Then I take a single drop of paint and place it somewhere into the riders body. That is my aid. The colour comes first where ever I touched my brush, then gently spreads to follow the water throughout the rider and the horse. The way the colour settles is never totally predictable or rigid. The colour is (probably) most vibrant where ever I first placed it, but it doesnt stay isolated there. It has flow on effects through both bodies.
Now here my metaphor starts to break down, because I'm not putting paint on a blank white, 2D page. I'm co-painting with my horse in shifting 3D landscape as we move through space and time - both of us rolling and tilting waves through the paper, adding water, dropping in pigment etc etc.
So each brush stroke I make is in response to the brush strokes my horse is making, the brush strokes each of us predict the other will make next... the canvas is changing and the next layer of the painting cannot be mapped out in advance.
Despite all that, I do think its really interesting to play around with noticing what happens when I place my paint in different places (lower leg, upper leg etc) at different times and trying to understand categories or trends in how this effects mine and my horses picture. Not planning each aid in advance, but noticing the differences between them in the moment.
And I also think its interesting to ask other riders about their experiences, experiments and what categories they are using when placing their paint in order to try get a broader understanding of how the art works in general.
And finally I do think its helpful to have lessons where someone suggests "what happens if you add a bit of blue to the armpit?" and "gosh did you just dab some red there? look at how the colour popped..." and "hows the water feel in the bottom right corner?" as long as that teaching are suggestions based in careful observation of the horse and rider moment to moment - not a memorised, routine checklist of heels down, shoulders back, blah blah blah.
Postscript: I'm still trying to square off my knowledge of how variable, fluid and in the moment very very good riding is with the equally valid knowledge that horses seem to get a lot of clarity, confidence and comfort from black and white leg means go rein means stop x = y program the computer to do as you say beep bop boop riding and training. I think we need both.