> you don't/can't "isolate" development of a muscle

This is not true at all. You may be confusing it with the assertion that you can't isolate individual heads of a particular muscle.

Yes, training has some systemic effect through hormonal activity, but muscle anabolism is very highly specific to local stimulus. Otherwise we'd all be just doing curls and watch our legs grow.

For a comically extreme example of localized functional muscle hypertrophy, search for the armwrestler Matthias "Hellboy" Schlitte :)

Yes, if you focus your training on one specific muscle, especially if it's a small one (arms basically) you will cause said muscle to be hypertrophied

But you can bet Matthias Schilitte doesn't stand all day training only said muscles (also he has a genetic problem)