From what I understand, it's not that they are integrated. Those apps allow to send and receive SMS. In doing so, they can simply send the queries via SMS on those apps, allowing to receive the result there.
To clarify - the system is in fact natively integrated across the SMS, Whatsapp and Facebook Messenger protocols. :) It's not using SMS as a way to talk to them.
If you use those, you'll see that the responses are almost instantaneous, compared to the lag which SMS inherently has. There is a general purpose API that can be used via any platform - Slack, Telegram, Gtalk, or email. Anything that supports text.
Edit: I was wrong!