[iraldir]

They often communicate through a central hub. If the hub is not supported officially, some middleware (hardware or software) can be implemented to make both device communicate in the same language.

Say I have an AC with just a remote control, and Alexa. They both don't know how to communicate with each other

A hardware device allows to simulate the remote to communicate with the AC, it thus become a connected AC. But that hardware still doesn't know how to communicate via alexa, it just gives me a phone app and web hooks to control the AC.

A software middleware then creates this connection, connecting to the webhook, and registering itself as a device on Alexa.

Now, this is all very clumsy and tends to break easily. Make it harder to maintain when you have like 10 devices with three middleware etc. But that's the IOT tech debt we have to accept until everyone agrees on the same interface, which might never come.

[greenz1]

Yeah!! I was facing this very issue. The comment by @schappim discusses more about this topic.