Question

How did radio companies such as NBC™, whose actual broadcasts were free, make money?

Answer 1

The answer would be that companies bought time from the radio network to run ads for their goods and services.

Answer 2

The model that allowed NBC to make money is pretty much the standard model used nowadays. The broadcasters were selling airtime to sponsors so they could run advertisements during breaks of the shows that were being aired. The more popular the show, the more expensive the air time. And it was found out that it was a great deal for both the Radio and the advertisers.