The long fight to make Apple's iMessage compatible with all devices has raged with little to show for it. But Google (de facto leader of the charge) and other mobile operators are now leveraging the European Union's Digital Market Act (DMA), according to the Financial Times. The law, which goes into effect in 2024, requires that "gatekeepers" not favor their own systems or limit third parties from interoperating within them. Gatekeepers are any company that meets specific financial and usage qualifications, including Google's parent company Alphabet, Apple, Samsung and others.
From what I've read, Google just owns the reference implementation. Apple could implement it themselves, but then lose out on certain non-cross-platform features, like e2e encryption.
I’ve read the specification. Google’s implementation is the only real implementation (raw RCS is basically a dead project) as Google have added a load of custom extensions to RCS that means, to be interoperable, you’d need to use Google’s (which I imagine requires licensing since it doesn’t appear to be open source).
That's basically what I said, but better.