Short answers, pulled from the story.
Popular music is a broad generic term for music with wide audience appeal, distributed through the music industry and accessible to people without formal musical training. Pop music refers to a specific genre within that broader category. The two terms are distinct and not interchangeable.
The original application of the term is to music of the 1880s Tin Pan Alley period in the United States. Tin Pan Alley was the name of the music publishing district in New York that developed new methods for promoting sheet music and songs to mass audiences.
The average BPM of popular songs fell from 116 in the 1960s to 100 in the 2000s. Songs also shifted toward minor keys, with around 85 percent of songs in a major key in the 1960s compared to roughly 40 percent today. Lyrics have become sadder, more antisocial, and more self-centered over the same period.
Robert Christgau coined the term semipopular music in 1970 to describe records that seemed accessible for popular consumption but proved unsuccessful commercially. He described it as "a cross-bred concentration of fashionable modes," citing albums like The Velvet Underground and The Gilded Palace of Sin by Flying Burrito Brothers as examples.
Artists are paid per individual stream, so longer songs result in fewer streams and lower income. The average song length in 2018 was 3 minutes and 30 seconds, which was 20 seconds shorter than the average in 2014.
By the 1970s, dominance in the recording industry rested with five transnational organizations: WEA, RCA, and CBS, which were American-owned, and EMI and Polygram, which were European-owned. In the 1990s, a further wave of inter-media consolidation saw music companies merge with film, television, and magazine companies to enable cross-marketing between subsidiaries.