The open protocol variant of that is called OMEMO. Same double ratchet and implemented using Signal's library (the same as used by WhatsApp) in many XMPP clients
WhatsApp paid WhisperSystems (the previous Signal company) for a commercial license. Same as Google. Dual-licensing their encryption protocol library was their main source of income back then.
AFAIK, even today you can fetch the WhatsApp Android apk, extract the zip and do a `strings classes*.dex | grep signal` to find class and package names matching the sources in the signal-protocol-java repository in the Signal GitHub.
