The feature only appears if both devices in the conversation have it.
I always thought of SMS as a bit like regular mail. (i.e. the message handed to the network with some addressing label with no feedback for the sender)
But this implies that there is some kind if handshake occurring between the devices, how does it really work?
Or they do it like their e2e-encryption: proprietary extensions to RCS without any compatibility with other RCS apps (not that there's any, given it's necessary to partner with Google to do so).
it only works if you both have the 'RCS' chat option enabled within your sms app. so if you switch your rcs to off, you won't be able to edit your sent texts and senders will not be able to change messages you've received. however, rcs messages are often faster than standard sms.