There seems to be some confusion going on in the comments above. Twitch didn’t build *any* of these experiences; third party developers did. All Twitch did was open up their platform for others to build experiences on it.
Doing this ALLOWS Twitch to focus on more core product features, and leaves the one-off, game-specific, experimental engineering efforts to external developers. Now Twitch engineers can spend more of their time on the core platform, stability, latency, user experience, and scalability.