I just used iMessage via satellite on the iOS public beta this past weekend. I forgot it even existed. The phone prompted me to update my location for "Find My" via Satellite, which I've done before. So I accepted, and got a screen that gave me the option of sending iMessage via satellite to. Sent one to a friend as a test. It's a pretty cool UI. It shows "Satellite" as the message type when writing messages, as you might see "SMS" or "iMessage" or "RCS". And then when you're in iMessage and connected, the dynamic island shows a green indicator. When not connected, the dynamic island is larger and shows you which way to turn to reconnect to satellite; presumably if you want to see if there are replies to your message, or whatever.