kittenzrulz123 My understanding is very general (I'm no software engineer and don't understand the technical stuff).
Basically, Google decided they wanted everyone to use RCS, so they pushed hard for it by providing the RCS service themselves. So all the carriers started using RCS, and Android users got RCS. Then Google got Apple to start using RCS as well, so iPhone users got RCS. Now that everyone is on board with RCS, Google has decided they no longer want to do all the work of providing the RCS service, so they've asked carriers to take over some (but not all) aspects. Some carriers have been faster than others in making those changes. AT&T and T-Mobile have both made changes, although there seems to be some differences in either how much they've done or how they've gone about doing it, I'm not sure. Verizon has not made these changes yet. That's why users here and elsewhere report that RCS works fine with Verizon and Verizon MVNO's, but user reports on AT&T are generally that RCS does not work and has not worked since the first breaking changes to Google Messages more than two months ago. T-Mobile users have reported that it works, but only if they install an older version of the app and even that workaround only seems to work for 2-ish days before they need to go through the cumbersome reinstall process again.
Because of these changes, Google Messages using AT&T/T-Mobile now wants some permissions that GOS blocks by default. As others have mentioned, GOS devs said a while back that a solution could be to provide an optional toggle to users to enable these additional permissions. (And I have seen a report on the GOS Discord server that a user was able to build their own version of GOS that allows these additional permissions and was able to get RCS to work with AT&T.) However, devs also said that giving Google Messages all these permissions is not the GOS way and that they are looking into a different solution. There haven't been any updates, as far as I'm aware, on either the toggle or the other solution.