Yes. The session keeps running when the screen is locked or the app is closed. You get a notification when each block ends and when the whole session is complete, so make sure notifications are allowed (Settings → Notifications → One Hour). Open the app at any time to see where you are.
While a session runs, a Live Activity shows the timer and a progress bar for every block, on the Lock Screen and in the Dynamic Island. Finished blocks are full, the running block is partly filled, and the next ones are empty. When the session is over it says “Session complete” and goes away the next time you open the app.
Go to Settings → Face ID & Passcode (or Touch ID & Passcode) and make sure Live Activities are allowed. Also check Settings → One Hour → Live Activities. Then start a new session in the app.
Open Settings → Notifications → One Hour and turn on Allow Notifications. The app asks for permission the first time you open it.
Tap the four dots icon (top right) when the timer is not running. Drag a block by the handle (≡) on the right. Tap the dots again to exit.
Swipe the block to the left and tap the trash icon. Blocks can’t be added, changed, reordered or deleted while the timer is running. Stop the timer first.
A session can be up to 60 minutes in total. Each block is at least 1 minute. Tap the dot on the right of a block to set its length. If your blocks already add up to 60 minutes, shorten one to make room for another.