The computing as to when they need switching on and off is done by the computer.
I used to use Tasker and Smart Life connected through Google to control my lights. It worked but it was fucking shit. I wrote all the logic for it based off my phone, so it kinda worked for me, and everyone else in the house hated it.
Now it’s all controlled by a computer based in the home, based on sensors that are in the home from all kinds of manufacturers using different protocols (because that’s what Home Assistant does, conglomerate protocols) it works much better for everyone involved.
You can try to make iot devices work without a computer or cloud if you like but it will just be absolutely garbage.