Google RCS isn’t the same as the carriers RCS, and the carriers support different versions.

So where is it used except Android? With any market share that makes it significant beyond android?

According to the sources I was able to find, iOS and Android collectively make up more than 99% of the smart phone market. There isn’t any significant market share outside of Android because there is no significant market outside of Android.

Nowhere because it requires a phone company SIM. No SIM no RCS. So no RCS between a tablet and phone or two tablets or a PC and phone. Also, ever so wonderfully, carrier provided RCS has no E2EE. So you need Google's app using their RCS gateway to have E2EE only between Google messages users.

That's a weird reply when a market has mostly two players. By definition there won't be any other significant market share.

That's my point, it maybe a standard but for all intents and purposes it is just used by a single player.

It's used by the manufacturers of phones, not just the Android base system. You don't need to be on the Google Messages app to use RCS. Eg it's supported on Samsung.