[dirkt]

* I don't know "what type of person uses this" in general, but I use it whenever I need to typeset music, which happens when I need to sing something with others. This includes be re-creating sheet music I have for better readability. I've never done any composing.

* It's not very similar to the source code. Basically you put in note names (c d e f ...) and durations (c4 e8 g8 c2). The rest is commands and markup to deal with the remaining very complex needs music typesetting has. Think Latex for music.

* I've never seen any git repos for that, but there are sites like CPDL and IMSLP that sometimes also include Lilypond source (or other sources).

* "How expressive is sheet music": I guess the easiest way to find out is to actually play an instrument or sing? It's a bit like asking "how expressive is the written word, like books and these things".