A:Answer Additional TVs can run their own AppleTV boxes, connected only by WiFi. You only need one AppleID to run multiple AppleTV boxes in your home. I only have one Comcast cable box in my home, to watch "live" news reporting (although a lot of that is available via various apps (CBS, ABC, MSNBC all have live stream; just not local news sources).
I have a TV in my basement, no cable box, just an older AppleTV box (2d edition box) that shows all my purchased movies, all the relevant apps and their programming (Netflix, Hulu, Acorn, etc).
Really it is the AppleTV box that is "Smart", The TV can be 15 years old, and show all the AppleTV can offer. Obviously, resolution will be limited. The AppleTV will show a 4K resolution movie at its maximum resolution, only on a screen that has enough pixels to show that.