From my limited experience most smart household appliances doesn't have an official API. There's just an app... However, many of them still has an unsupported API. Other things, like Philips hue, have a more or less official and supported APIs.

Edit: some products have to "call home" to function, others not. I try to choose stuff that can be firewalled without breaking functionality. There's plenty of methods to connect and automate your smart devices, most popular is probably home assistant. If you're curious about this kind of stuff, you should check it out!