OBS Browser Source overlay - any modern OBS install, add a Browser Source with URL http://localhost/overlay/index.html?match=YOUR_MATCH. Lives at observer-api/overlay/.
Twitch Extension panel - boilerplate at /samples/twitch-extension/. Renders the live scoreboard inside Twitch's extension panel.
Webhook bridge - POST to /api/bridge/subscriptions with your OBS WebSocket bridge, vMix Web Controller URL, Streamlabs, Streamer.bot, or your own HTTP endpoint. We POST every event envelope your way with header x-rail-bridge-source.
YouTube Live / Kick - chat APIs are out of scope for the rail; use any community Node client (yt-watchparty, kick-chat-bot). The event envelope drives chat commands.