Fire OS is the operating system that runs Amazon's Fire TV and tablets. Fire OS is a fork of Android, so if your app runs on Android, it will most likely run on Amazon's Fire devices too. You can install most of the apps on Fire TV no matter whether it is available on Fire TV App Store, following below the tips on https://forums.tomsguide.com/faq/how-to-install-and-run-your-apps-on-fire-tv.203525/
Installing and running apps on your Fire TV is not as hard as it might seem. You can install and load any kind of app on your Fire TV device, even if they can’t be found on the app store. Here’s how you can get any kind of app installed and running.
Installing and running your app from the Fire TV App Store
1. Browse apps
Check Fire TV’s main menu with your remote control and scroll through the apps available on your device. Select the app you want.
NOTE: There are some apps that require payment. If you have a payment method already linked to your account, funds will be debited when you select that particular app.
2. Install the app
After finding your preferred app, install it by pressing the ‘select’ button. Then press the ‘Get’ button for installation to begin.
3. Launch the app
Once the app has been installed, the button will change to ‘Open’. Click this button to get your app running.
Installing apps that are not available on the Fire TV App store
1. Turn on Settings
If you are installing an app that is not available on the app store, you need to go to the home page. Navigate to 'Settings', highlight ‘Device’ and then ‘Developer Options’.
Select ‘Apps from unknown sources’ and when the warning screen appears, select ‘Turn on’. This will allow you to install apps from outside Amazon.
- Install Downloader
Go to the Fire TV home page, then use the search tool on the left to search for ‘Downloader’. This app will help you download other apps. Open the installer window as in step 2 above (Installing through Fire TV), then launch the app.
- Navigate to APK Site URL
Once done, you can navigate to the Home page and enter the web URL for your APK files, or navigate to any site on the web that has the APK files. Once you select the files, the ‘Downloader’app will start the installation process. Just select ‘install’, to start and then ‘Done’to complete.
Your app will now appear on the home page under ‘Apps’. You can click it to run the app on your Fire TV device.
As a developer, You can quickly check your app's compatibility with Amazon through the App Testing Service. then you might not have to adjust your Android code at all to publish your app on Amazon's platform. details refer to https://www.developer.amazon.com/docs/fire-tv/installing-and-running-your-app.html