CWA was using the Exposure Notifications from Apple and Google[0].
Exposure Notifications is not an app, but a framework and a protocol, and CWA was an implementation of it, hooking in the features developed by Apple and Google for iOs and Android
Ah, I was under the impression Google and Apple had created their own separate apps and these third-party apps merely worked on the same principle. It's interesting that the document you linked indicates the interoperability between different countries' apps was down to an EU-wide standard. I guess this meant the apps of different EU countries were compatible but only with each other.