So… how did musicals come about? Here’s a great short introduction that takes us from 2500BC to the present day…
The below documentary (from a British perspective) is an excellently detailed introduction to this form. Aimed at the general public, this is quite easy to understand.
MSU have put their whole Introduction to Theatre and Drama Arts 101 course online and below are three hour long lectures that go into great detail on the history and theory of the musical form (it’s a very American perspective). This is followed by an hour long interview with Michael Brill, a musical theatre practitioner. You could jump straight to that if you want an excellent summary of how musicals work in practice.