Short answers, pulled from the story.
The primary focus of the popular music industry in the late 18th and early 19th centuries was the commodity form of sheet music. Inexpensive and widely available sheet music allowed amateur, middle-class music-makers to play and sing popular music at home, often centering their efforts around the piano.
Radio broadcasting of music began in the early 1920s and helped spread popular songs to a huge audience. This technology enabled a much larger proportion of the population to hear songs performed by professional singers and music ensembles, including individuals from lower income groups who previously would not have been able to afford concert tickets.
Popular music has gotten slower since the 1960s, with the average BPM of popular songs from the 1960s being 116. The average of the 2000s was 100 BPM, and songs in the 2018 Billboard Top 5 were between 3 minutes and 21 seconds and 3 minutes and 40 seconds long.
DJ Kool Herc is known for creating hip-hop itself in the 1970s in the contemporary United States. He used the technique of mixing two identical records back and forth to make unique-sounding sounds that later gave birth to rap.
Form in popular music is most often sectional, with the most common sections being verse, chorus or refrain, and bridge. The verse and chorus are considered the primary elements, with each verse usually having the same melody but changing lyrics, while the chorus or refrain usually has a melodic phrase and a key lyrical line which is repeated.
The 1990s brought digital equipment and software that democratized recording technology, leading to an explosion of diverse voices. The ubiquity of streaming changed the economics of music consumption as artists are now paid per individual stream, leading to a trend where songs have shortened again, with the average song length in 2018 being 3 minutes and 30 seconds.