Google uses Signal protocol to handle E2E for RCS communication: https://arstechnica.com/gadgets/2021/06/google-enables-end-to-end-encryption-for-androids-default-sms-rcs-app/

The way I understand RCS (and that someone will correct me) is that it's similar to the OSI network stack. RCS is like the lower transport or network layer for routing messages between providers, so when you send a text on AT&T it can be delivered to a Verizon phone via RCS as opposed to SMS. Within the messages, the data can be encrypted using the Signal protocol to do the key exchange and whatnot necessary for the communication.

When you're sending an iMessage to someone, it's not going through the cell provider and instead using the data connection to send the message to Apple who delivers the message. When iMessage falls back to SMS, that is going through the cell carrier, and had technical limitations RCS tries to resolve, including not being encrypted. Realistically, having this fallback not be over SMS but via RCS is the only option, since apple will never get rid of iMessage. Short of legislation, once Verizon or AT&T announces they're going to stop supporting SMS, that'll be when Apple takes RCS seriously, because there will be no other option.