The buttons are not universal in the different channels/apps. The developers of the channels/apps can assign different features to any of the available buttons.
The instructions (* button) are for the Roku UI and are integrated with the Roku Channel. Of course, the developers are free to use this mapping as well in their channel/app development, but they do not have to. Additionally, it is mentioned in those developer docs that developers are restricted from using the * button "during video playback". Reserved for the Roku UI. (cannot be used for another feature/action).
Do not design your channel to use the Star/Option button during video playback.
For Prime Video, in order to get to the closed captioning, you need to press the Down arrow to bring up a Menu Info Overlay, and then use the Up and Right arrows to select Subtitles.
Some other channels/apps use a similar procedure to toggle this Menu overlay.
In this manner, they can override the Roku defaults during the channel/app usage without affecting the actual default Roku defaults in the UI, thus affecting other channels/apps.
