If you're a big fan of Alexa, you can make her your assistant for your Android phone. Bummer part is she won't activate if you say "Hey, Alexa!" but if you do the rigorous action of holding down the home button for a few seconds to activate her, you'll be good to go.
If you want to switch to Alexa, just go to Settings>Apps>Default Apps>Assist & voice Input. You should be able to choose between Google Assistant, Microsoft's Cortana, or Amazon Alexa (but you need the apps for Cortana and Alexa on your phone for them to be accessible.)