just to qualify the above (slightly)
I started of in modular because about 7 years ago* I was staring at my moogerfoogers and cp251 and realised that they were effectively part of a modular synthesizer - so I started researching modular synthesizers - about 2 years later I started building one
*this was over 30 years after first picking up a guitar and 20 years since I got into synthesizers/music production - although there have been a few gaps due to life!
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities