How do they know how many people watched Joe Biden v how many watch Trump?

Here ya go:

https://www.nytimes.com/2020/10/16/business/media/biden-trump-town-hall-ratings.html

Nielson ratings on town hall meeting

I was tapped to be Nielson family one year. We had to keep record of what we watched on TV for, like, 2 weeks. Then I had to mail it in.
So were the Nielson families checking in via the internet?

They have a little box now in your house connected to the internet that transmits what you are watching now.

And my friends. Let me introduce you to ACR. There are multiple brands of smart tvs (and Rokus) that send back frames of what you are watching for advertising purposes. This is why I go out of my way to buy a dumb tv.

So, does it capture people who watch on DVR? Or, only people watching in real time?

@JLeslie I think both. They mentioned in one of the news channels that a lot of people taped one to watch later and watch one and that some people kept flipping from one to another.