you want a device that is always on and listening for "Hey Siri Please turn on the living room lights" when you walk through the door. You're not going to keep a television on to do that.
That's siri, not homekit.
You can't talk to the TV, as it doesn't have siri.
And just because the screen is off, don't think the TV isn't powered up. The electronics are probably still energized in the background, so a TV could be always listening, even it it's turned "off".
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Homekit lets you control your TVs power, and select inputs.
For instance you could make a scene that you use when you game. It could turn the TV on, set it to the correct HDMI input, adjust the lights to a lower levels, and close the blinds.
or as part of a goodnight scene which turns the TV off.
you can also automate things, if everyone in your house has an iPhone, and someone is bad about leaving the TV on when everyone leaves, you can make an automation "when the last person leaves the house, turn off the TV"
Homekit TVs will also tell homekit when they turn on/off, so you can make automations based on that... "when TV tuns on, dim the lights"
I have an LED strip behind my TV for a bias light, It turns on an off via homekit automations.
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Airplay will let you send audio/video from another apple device on your network. So you could mirror your Mac or iPhone's screen on the TV.
One thing about airplay, things are either receivers or transmitters, and occasionally both.
Phones and Macs are transmitters
appleTV and HomePods are both transmitters and receivers. (although they only transmit audio, not video)
third party speakers and telvisions are receivers only.
So don't think, oh... "I have a TV, and Sonos speakers that both have airplay, I'll just send audio from the TV directly to the speakers using airplay" That won't work as they are both receivers.
But, since the AppleTV is also a transmitter, you could send audio from an AppleTV to a Sonos speaker via airplay.
(there are third party apps for various devices that will let you send or receive, but I'm talking default behavior)
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then there's the
AppleTV+ app, which is a streaming service, like Netflix or amazon prime.
It lets you access Apple's content, And if you've subscribed to other services through apple, you can also access some premium content like HBO or starz.
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for the most part, if a TV has one, it
probably has the others, but not guaranteed, make sure you read the specs if you really want one specific feature.
If you've got an AppleTV, you already have airplay and appleTV+, so having those in the TV is not really necessary.
