I have personally received Nielsen Ratings, even as late as a few years ago. Their web site says:
Electronic and proprietary metering technology is at the heart of Nielsen audience measurement. In addition to capturing what channels viewers are watching on each television set in the home, our meters can identify who is watching and when, including “time-shifted” viewing—the watching of recorded programming up to seven days after an original broadcast.
Chosen at random through proven methodology, Nielsen’s U.S. TV families represent a cross-section of representative homes throughout the country. We measure viewing using our national and local people meters, which capture information about what’s being viewed and when, and in the major U.S. markets, specifically who and how many people are watching. We also have TV set meters in many local markets, and we collect more than two million paper diaries from audiences across the country each year during “sweeps”—specific periods during the months of February, May, July and November. To measure video content viewed on mobile devices, we have expanded our panels to incorporate census-style data from third parties in order to capture the breadth and depth of consumer usage.
Nielsen delivers a constant, real-time stream of information, revealing tuning behavior during programs and commercials. We can tell clients which commercials are being watched, as well as which ones have the strongest engagement and impact. We analyze which position in the program or commercial block is most effective for a specific brand and which markets will create the best return on investment for brands.
I know Nielsen is just one ratings company, but would bet the other ones are pretty much the same.