Where to break the ice?
After thinking about it long enough - I think the only way to really get started on any development (per the main body of the software or plugins) - is to just go to github, download the code, and then just dive in.
That said, it usually makes the most sense to work on some thing or some feature one is interested in tweaking. MuseScore is a pretty big program, so navigating to the right areas of the source code, and identifying all the requisites and dependencies seems to be the next part.
I'm expecting a "no" to my next question, but is there some way get a lay of the code's landscape? Like either a written or a human in-the-know guide?
Check https://musescore.org/en/handbook/developers-handbook and you'd find https://musescore.org/en/developers-handbook/references/design-implemen… and https://musescore.org/en/developers-handbook/references/musescore-inter…. Not much, but maybe a start?
And you can join IRC anytime and talk with other developers: https://kiwiirc.com/client/irc.freenode.net/?nick=musescore|?#musescore