Want Alexa to be the default virtual assistant on your Android phone?
To arrange this, go to Settings > Apps & notifications > Default apps > Assist & voice input and click on "Amazon Alexa" (see image at right). There a few things you should know. If you have an Android phone that allows you to open Google Assistant with a squeeze, this will not work with Alexa; she is non-squeezable, sort of like Howie Mandel. And hotwords won't activate Alexa either. Sure, you can scream at the top of your lungs, "Alexa, Alexa, ALEXA!!!!," but it will be a waste of your breath. You will need to long-press your home button to get her attention.
Just in case your crush on Alexa turns out to be a passing fancy, no worries! You can have Google Assistant return as your default virtual personal assistant by going back to the steps listed in the first sentence of the above paragraph, and instead of clicking on Alexa, click on "Enhanced Google services."