Maybe I'm wrong, but I don't think either Google or Amazon is providing their digital assistant to be installed on any device.
Google Assistant can be accessed on any device via their Allo app. I think my original comment already mentioned the numerous third party apps providing Alexa to an Android or iOS device. Cortana can be downloaded. Samsung provides S-Voice on their devices. My point was, and still is, you can replace the voice recognition assistant that came with your phone.