It's all harmony, composition, polyphony, form and analysis of works, but

Harmony is the harmony of the masses.

Composition is a popular composition.

Polyphony is a chord.

Music form is a popular music form.

The analysis of works is the analysis of popular works.

Moreover, the classical school was studied before it became popular because it was the inevitable route of music development, just like the classical Chinese which has now become vernacular. Because classical Chinese is difficult to understand, but people want to communicate, so they are forced to naturally develop into vernacular Chinese, which is the popular mainstream language!

The classical period also gradually evolved into a romantic period, and modern music is atonal. As long as you learn to compose music, you will naturally go from classical to modern and then to contemporary!