Situation 01You are not sure which app is the right one
App stores contain a lot of things with Alexa in the name. Exactly one of them is the app published by the device manufacturer, and it is free.
Four checks, all visible before you install anything. The developer name on the listing should be the device manufacturer, Amazon — not an individual or an unfamiliar studio. The install count should be in the hundreds of millions; a listing with a few thousand is not it. The price should be nothing. And the screenshots should show an actual interface rather than stock photographs of speakers.
Where to get it. The store built into your phone — the App Store on iPhone and iPad, Google Play or the manufacturer's own store on Android. Nowhere else. Installation files from websites, forums or file-sharing services are a genuine security risk, and that is not how this app is distributed. We do not host or link to installers, and neither should anyone else.
A note on cost. Setting up a device you already own is free. If anything asks you to pay to complete setup, to install a second app first, or to buy an "activation" of any kind, it is not part of this process.