Short answers, pulled from the story.
Scholars define popular music as songs known to listeners mainly from hearing them rather than reading sheet music. This definition distinguishes the genre from classical music where musicians traditionally learn pieces through written scores.
The most significant feature was the focus on the commodity form of sheet music which made it possible for amateur middle-class music-makers to play songs at home. Inexpensive widely available sheet music versions allowed people to use instruments like the piano to play melodies chords and basslines simultaneously.
Songs have shortened again partially due to the ubiquity of streaming platforms with the average song length in 2018 being 3 minutes and 30 seconds. This duration is 20 seconds shorter than the average in 2014 because artists are paid per individual stream so longer songs could mean fewer streams and less revenue.
Political songs became an important category during the struggle against colonial rule to boost citizens' morale based on Western marches and hymns reflecting European education systems. Activists used protest and freedom songs to persuade individuals to take action and become educated with the struggle using traditional tribal styles along with hybrid forms of imported genres.
Dangdut formed from two other genres indo pop and underground music coming together to create a new fusion sound that takes noisy instrumentation but makes it easier to listen to. This genre has formed into a larger social movement including clothing youth culture resurgence of Islam and capitalist entertainment industry.