Pretty much. RCS just defines the protocol. That is basically how messages are formatted, how recipients and senders are labeled, how timestamps are formatted, how messages are compressed, transferred and possibly stored and resent in case of inability to deliver (e.g. recipients phone is off) and a lot more technical stuff.

To use this, you need a network of servers that can actually receive messages from phones/ cell network, then possibly transfer them to a server/ cell tower near the recipient(s) to have it broadcast there, making sure only the intended recipient can decrypt it, and possibly storing it for an unknown duration and a number of attempts to deliver it if the recipient is unreachable. And possibly sending a receipt or failure-to-deliver message back to the sender as well. And all of this has to work reliably 24/7, because people will not accept downtimes.

And then you also need client software on the phones to send/receive messages using the RCS protocol as well, hopefully handling true e2e encryption as well.

It's a lot of hardware and software engineering to actuallyimplement such a seemingly fundamental standard.