AndroidTV which replaces all Google services with Amazon's own services.
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Since it is based on Android you can sideload any Android app.
Pro Cheap devices
Amazon FireTV devices are cheap and have often a discount.
There are plenty of ads for Amazon Prime or products.
Con Amazon account
Needs an Amazon account.
Con Focuses on Amazon content
The main purpose of the stick is to provide a portal to Amazon's various online services.
The UI is proprietary software.