Over the Air audience measurement is still very much alive for AM, FM and TV.
Inaudible to humans, signal codes are embedded into the audio at the studio, and decoded by devices (now phone apps) carried by the persons who are part of the sample. A properly randomized set of sample households (or persons) can provide pretty accurate survey results with a relatively small sample - say a thousand devices in a metro of 1 million. This is how ratings for shows were done pre-internet. They also used to use paper diaries for sample members to record their watching/listening during a specific week or two period. Those ratings drove the cost of ads on/near a particular show, which air personalities kept/lost their jobs, which shows were renewed/canceled, etc. The ratings still do that, but the technology for data collection has changed greatly.
With connected cars and TV's, and monitoring apps on the phones of those in the sample, its all done in real time today, unless you are in a very small market, where paper diaries are still in use along with the newer technology. Your connected TV and car pay attention to what you listen to, and report it to the companies who compile ratings. They now have data to the second including what, where & when and how long you watch/listen. The data aggregators also sell that data to marketing folks so you get "targeted" ads. We've all become test subjects.
When I worked at a "big three" network, we received reports each morning telling us which affiliates ran/didn't run what specific promo for a network show/event (and when) the day before, for each of the >200 affiliates across the country. We used those reports for affiliate contract compliance. - all based on embedded inaudible data in the audio.