The video streams are delivered from their OpenConnect appliances. The video encoding, their actual website and all the client interaction is run in AWS, active/active in three regions (and multiple availability zones per region).

The AWS part is also very dynamic, at any given time most customers are (unknowingly/behind the scenes) participating in 8-10 beta features.

That said, this is all based on talks and presentations they have given at various conferences in the past. It could be different, especially some AWS parts.