The biggest advantage of Android—and, frankly, the reason I switched to the platform over iOS back in 2012—is its tremendous freedom of choice. You typically aren't locked into using specific apps or services. Whatever app you want to use for a given function can be your default, and third-party apps aren't treated as second-rate. Today is further proof of that, as you can now set Amazon's Alexa as your default assistant—even over the Google Assistant if you so choose.
Last year, Microsoft was one of the first companies to jump onto the assistant default settings, enabling you to set Cortana as your default assistant (if, for some reason, that's something you wanted to do).
Alexa activated by long-pressing the home button, once set as default.
There are a few caveats to setting Alexa as your default assistant, though. For one, if your phone has a squeezable gesture to activate the Google Assistant, like the Pixel 2 phones, it won't work for Alexa. Hotword detection also isn't available, so you'll be long-pressing your hardware or software home button to trigger Alexa.
If you'd like to see what Android is like with Alexa as your default assistant, it's easy enough to flip the switch. Just go to Settings -> Apps & notifications -> Default apps -> Assist & voice input (or just search "Assist") and select "Amazon Alexa" as your new default. That is, assuming you have the app already installed from the Google Play Store.
Once Alexa is set as your default assist app, long-pressing the hardware or software home navigation button will bring up a listening dialog for the app pretty much as you'd expect. Reversing the change, should you like to go back, is as easy as going to that same location and selecting "Enhanced Google services" instead.