Accessing the keys from the server isn’t really a mystery or hidden. It’s technically possible for Apple to write software to query servers run by Google as well as any servers they created for themselves.
You don’t need implementation source code when you have open standards already.
WhatsApp actually used Signal’s development team to rollout the Signal protocol for them, but that app is still untrustworthy.
Here are the links to the documentation for these standards:
MLS - https://datatracker.ietf.org/wg/mls/documents/
Signal Protocol - https://www.signal.org/docs/
RCS - https://www.gsma.com/futurenetworks/rcs/universal-profile/ & https://www.gsma.com/futurenetworks/resources/rcs-up-2-4-uni/
@skullgiver