Google provides a client and infrastructure, which they sell to carriers and which has a number of proprietary extensions, including E2EE if the message is Google to Google. If a carrier does not provide Universal Profile, Google provides it. If you send a message using Google Messages, it may default to Google's profile, which is not open and only available to Google Messages users, which is arguably no different to iMessage. Apple provides an RCS client which relies on carrier infrastructure. If there is no compatible profile AIUI, it falls back to MMS or fails.