Alexa can be used as Android's default assistant application if you prefer it over Google Assistant for all your smart assistant needs. The capability was discovered by a user on Reddit who displays that it's possible to select Alexa as the app for assistants from the settings menu where you can change this function, though it's important to remember that you'll need to have the Alexa app installed on your device for you to to be able to switch it out. That said, Alexa on Android devices is not as powerful as Google Assistant as it's not tied into as many things so it will be a little more limiting should you choose to use it. It's also worth noting that it may not be configurable as the default for all Android devices that can install the Alexa app.
If you already have Alexa installed on your Android device getting to the page where you can set it as your default assist app is pretty simply though there are a few steps to go through. First, after installation head into the Settings menu, then scroll to Apps & Notifications and enter this sub-menu. From there, scroll and tap on Default Apps, and this is where you will find the Assist App menu, which is where you can find the option to switch out Google Assistant for Alexa, or simply set it to none if you'd rather not have one at all.
It's not exactly clear when this option started popping up as it wasn't always possible to set Alexa as the default assist app on Android even after the launch of the app on the platform, so it could have been available for a while or it may be a new compatibility function that was just recently added. Nevertheless, the option is there now to switch to it for those that do like Alexa better than Google Assistant, and while it may not be as functional yet, there's always the chance that it will gain more capabilities for Android devices in the future.