It has its own server-client format that replaces both the ssh / mosh layer and the tmux layer, allowing the iOS app to connect to existing sessions on the computer (also the Mac app can either connect to the local Rex server that is included in the app, or to remote servers over Tailscale).

It doesn’t replicate the “remote authentication” part of ssh, and leaves that to Tailscale instead.

Here’s a demo of it on iOS. It’s on a Duo, but you can get the idea from the single-screen part of the demo.

https://xxcancel.com/almonk/status/2114390982049329442#m