Mobile projects get a Phone tab beside Preview. It shows your app the way a phone sees it, a QR code to scan with your phone's camera, an Open in Expo Go button for when you are already on the phone, and one line that says whether the phone preview is running.
Your dev machine now has its own address. Anyone you give it to can use the app while the machine is on: your phone, a teammate, a client on a call, a webhook you are testing. The machine stays awake while the app is in use from any device.
Only you can turn it on: opening the project in Playcode wakes it, and so does the agent while it works for you. It goes to sleep after a quiet while, and after four hours without you or the agent. When the app on your phone finds the machine asleep, it shows a banner with one button that opens the project and wakes it.
The address is not a public link. Publishing still is: a published app keeps its permanent link that works for everyone, whether your dev machine is awake or not.