Thank you, those are exactly the kinds of music typesetting/engraving software (and notation syntax) I was curious about.

Following a few links, I found a great overview:

MusiXTEX - Using TEX to write polyphonic instrumental music

http://icking-music-archive.org/software/musixtex/musixdoc.p... (pdf)

For Lilypond, here's an introduction that demonstrates how to "write music by typing text":

Lilypond: "Compiling" Music

https://lilypond.org/text-input.html