...
Anyway, the modern idea of a pop song came with the invention of recorded music, around the end of the 1800s. There was a place called Tin Pan Alley, and a building called the Brill Building, where workers would go to offices, and write songs, every day, 9-5.
Even before recorded music, actually, there was the trade of written music for the middle classes, who were not culturally sophisticated enough to want exclusively to play more “serious” music - Chopin, or whatever. Or perhaps not technically skillful enough.
It’s hard for us, today, to understand, that before the modern age, people made their own entertainment. Women would collect in one another’s houses, and do readings from plays or poems. People would gather in their drawing rooms, and make their own music. Or at least play it - often, using sheet music, often consisting of songs from the nascent music business...
But with the invention of records, a strict format began to take shape. The earliest widely available commercial recordings were on wax disks - cylinders had not made much headway, were bulkier to transport, and had a lower sound quality. The first disks were breakable and only one-sided, and revolved at 78 RPM - how or why that number was arrived at, I’ve no idea! (I could Google it, I suppose, but I don’t really care!) The point being, they could only hold about three minutes of music, and so the modern idea of the three minute pop song was solidified.