I found out that I can (as a workaround) view Amazon Prime Videos via the Oculus Go web browser … really cool! When the video is still showing the loading/buffer animation I can switch to full screen in the top right corner. After the video has started playing I do not see these options anymore.
Is there a way to bring them up when the video is already playing?
You should be able to pull the trigger on the controller while in the browser full screen view. A bar will appear near the bottom of your field of view. You can click an icon there. I think these buttons need a tutorial, since clicking the wrong one for the type of video you're watching can be super disorienting.