WhatsApp has been forced by the EU to provide access to third parties. If there's any app that third party apps can interact with, it's WhatsApp.
I also can't really find the mechanism at use here. I don't know if WhatsApp is exposing some kind of dedicated assistant API that an alternative assistant (the one you can pick in the settings) might be able to use.
I'll need to check again. Maybe it's my rusty Android skills, but last time I checked (circa a year ago), I got the impression it's going out of its way to stay fully opaque from the outside.
It's also notoriously the one popular app that intentionally doesn't offer any kind of API access for normal users - it only has one to allow companies to automate advertising bots.
I also can't really find the mechanism at use here. I don't know if WhatsApp is exposing some kind of dedicated assistant API that an alternative assistant (the one you can pick in the settings) might be able to use.