It's a mystery to me--how do they know what channel I'm watching? Does Big Brother tap into my cable box?
THIS IS WAY OFF-TOPIC, but
The cable company almost certainly does. I believe they also know what you search for.
How else are they to be able to guess what shows to recommend, if you ask for recommendations?
Plus, that info can be sold, so they probably do.
If your cable box connects to your TV through an HDMI cable, they can also tell what type of TV you have, and whether it is turned on.
But if you share your house with several people, I'm not sure if they can distinguish who watches. In theory, that could be done by monitoring your remote key depression rhythms, something that has been done on computers for decades.
Your cable company can re-program your cable box or DVR, which could give them access to anything the cable box or DVR can access, if they want it, including keypress rhythms.
A while ago, there was a big fuss because many high end Samsung TVs have built in cameras and microphones (nominally for teleconferencing), and, by default, anyone on the Internet could access them. I think Samsung finally provided a way to turn that off, but few people know about it. I have no reason to assume that cable boxes do or do not contain such devices. It would add to the cost, so they probably don't.
If you access your cable company's on-demand or other content through a computer (or streaming or mobile device, which are computers of a sort) they probably have access to many things on that computer, including cameras and microphones.
I've mostly stopped worrying about big brother, and little brothers like the cable company or ISP. It's too hard to keep them out. Worry about the privacy of your financial info. Worry about built in cameras on TVs and computers and mobile devices, if you use them naked. (Cover the cameras!) Otherwise, I suggest you forget about it.