This is from Google and there is more detail there if you want it:
"TV ratings underwent a massive shift in 2025, driven primarily by
Nielsen's implementation of "Big Data + Panel" measurement, which combines traditional panel households with data from millions of smart TVs and set-top boxes. This shift generally resulted in lower ratings for many networks, particularly in the prized 25–54 demographic, despite aiming for higher accuracy.
- Sports Measurement Improvements: While the overall trend for linear is down, the new "Big Data + Panel" system actually improved ratings for sports by capturing more viewers than the previous panel-only system, note The Hollywood Reporter."