There’s two in-app browsers experiences in Android. Developers can either make a pretty facade to a WebView like Facebook or Instagram, that one you have the capabilities to inject JS or CSS as needed, like here. The other one is Chrome Custom Tabs, which browsers like Firefox and Chrome support, developers don’t have access to your data in here

I remember reading that it supports mouse and keyboard, but the main input control are hand motions

A NAS or an old PC for hosting Plex and I’m not sure if Nvidia is still updating the Nvidia Shield TV, that one was one if not the best Android TV box