“A radio” refers to a specific receiver or transceiver.
“The radio” refers to the entire ecosystem of transmission towers, broadcasting equipment, regulations to prevent overlapping signals, the people or companies transmitting, and the content being transmitted.
It’s in the same sense where if a good or service is sold on a market, there might only be one place to buy it, but if it’s sold on the market, it’s available for wider purchase.
In this instance, the word “the” is used to make the concept all-encompassing, not to limit it to a specific instance.
