Choosing the technology path
Fire TV has historically run Fire OS, an Android fork, so established apps are native Android written in Kotlin or Java, or web apps wrapped through Amazon's web app tooling. Amazon has also been moving newer devices toward Vega OS with React Native, so check which operating systems your target devices use today. Existing Android TV apps are a good starting point, because Leanback and Compose for TV patterns carry over. The work lies in replacing Google Play Services, handling different device capabilities and passing Amazon's own tests.
If you already have an Android TV app, much of it can be adapted to Fire OS, but Google services are absent and Amazon APIs replace them. We audit dependencies for Play Services calls, such as billing, cast and location, before estimating the port. Device spread is the second hazard. Older sticks have less memory and weaker decoders than the newest, so we keep an agreed oldest-supported device, profile on it often and avoid assumptions that only hold on premium hardware.