The difference between AI, machine learning and digital assistants

Microsoft, Google, Amazon, Apple, Facebook and others are aggressively imbuing their products and services with "intelligence." We're told that computer systems will become smarter, will anticipate our needs, proactively serve us, and understand us in various contexts and via multiple methods including voice, text, inking and motion. The digital "face" of this promising tech comes in the form of assistants like Cortana, Assistant, Siri, Alexa, and Bixby. As smart speakers and the AI genies they harbor begin finding a place in our homes, and as those same digital sprites serve us through our phones and PCs, the promise of an AI-assisted existence is becoming a reality. But what's behind the magic of our speaking, "Hey, Cortana," and her - its - response?