Electronics engineering (all engineering as far as possible really) does have a good amount of modularity. So understanding a circuit is about understanding each module, or simply being told their general function. You have a general geometry e.g. a balun or an antenna should follow, but the details very much follow from complex electromagnetic interactions, and I assume are designed through a intuition, simulation and testing loop. So if another engineer needs to slightly modify your design, he will usually just try adding or modifying individual modules to get the desired function -- that could include modifying the geometry of a circuit board analog filter, re-shaping an antenna for different directivity characteristics, etc.
