The protocol is open but key exchange is not: even on Android, third-party messengers can’t interoperate with Google Messages. See page 11 of that PDF.
I also doubt that Google would withhold such details from parties like Apple even if they maintained their proprietary format. Small developers would need to reverse engineer the details, unfortunately.
If they seriously wanted random third parties to implement it, anyhow, they'd have published a specification, not an "overview"
("Only Messages was allowed to support it. At the time it was written, for example, it was years after the Signal developers had asked for basic RCS support (2015) and given up on Google relenting and allowing it in 2018")
At this point I think they just wanted to get rid of their SMS backup feature and used RCS as an excuse.
Put another way, if this was a Signal thing what other Android app is doing this?