How do virtual assistants work? [D]

How do virtual assistants like Siri, Alexa, Bixby, Cortana, and Google assistant work? I have found some things searching how Google assistant and Siri work, and this book on Google books: using Google scholar but besides the book I have not been able to find how they work and when I do the diagrams and descriptions seem to be quite vague and generalize a lot like grouping components into boxes in diagrams. Or they seem to be too specific for a niche.