Let’s start with saying that the so called AI is not “intelligence”. It’s just a pretty advanced algorithm doing some “magic”. Behind the scenes it’s just a regular linear algebra equations dealing with bazillions of rows and columns feeded from the huge matrix. Modern mobile devices are capable of running “AI” models locally but it would cost a lot of power which would make a phone as unusable as Apple watch (aka charge it few times a day). So, the easy tasks like face id are done locally, more complex one (voice assistants) are outsourced to the cloud.