Most of the features of the Nest Protect smoke alarms don't use voice commands. You will use the app and control it from there. I don't have Alexa but use Google Assistant but there is never any reason to use anything other than the app to to control them.
See Beginner's guide to the Nest Protect smoke and CO alarm - Google Nest Help and Control Nest products with Amazon Alexa - Google Nest Help . Those links give you a bigger picture of what you can do with the products.
Hope that helps!