I don't know which part you're most interested in, but in a nutshell: the events are fetched from the same API as the Apple TV Events app (found that using a proxy), the Chromecast streaming uses Bonjour to find Chromecast devices on the network and the CASTV2 protocol over TCP to send commands to them.
The Chromecast protocol specification (CASTV2) itself is sort of open. To implement it on macOS I did a little bit of reverse engineering and used a description of the protocol I found here: https://github.com/thibauts/node-castv2