I mean, from a technical perspective. How does the assistant know that it should send the request to IFTTT servers?
When I created my own actions on Google Assistant I used the actions SDK and uploaded a manifest file into my account. This manifest basically maps all the phrases to intents and then to a predefined fulfillment server. I get that part. The thing is though, that I was only able to trigger my app by saying something like: "Ok Google, talk to my app". With IFTTT this doesn't seem to be necessary. A user can just say something like "Ok Google, mow the lawn" instead of "Ok Google, tell IFTTT to mow the lawn".
So how does that work behind the scenes? Is that Google explicitly whitelisting a service here? Or what part am I missing?
So I actually got in touch with Google Support and apparently IFTTT is also a Google company. They can make use of some private API and get priority when it comes to implicit intent invocation