A Mobile project's Notification block shows a real, native device notification, but it's a local notification, fired immediately by an action (a button tap, a Timer tick), entirely on-device, no server involved.
What this means in practice
Notification can only react to something happening while your app is open. For a notification a SERVER can send at any time, even while your app isn't running at all, see "Real push notifications on Mobile (Firebase)", that's a separate, newer block.
Scheduled Reminder: a notification for later, still on-device
The Scheduled Reminder block is different: give it a specific date and time (once) or a time of day (daily), and it fires at that moment even if your app isn't open, the device itself keeps track of it, the same way any app's local alarm/reminder works. It's rearmed every time the app is opened (so it survives a device restart without needing anything extra from you), and a past "once" date/time is simply skipped, never fired retroactively. It isn't an exact alarm, delivery can be a few minutes late under aggressive battery optimization, fine for "remind me around this time," not for anything safety-critical. It's still entirely on-device: no server is involved, and it can't be triggered remotely.
Timer also runs while the screen is open
A Timer block on Mobile only keeps running while its screen is open, unlike Desktop's equivalent, which runs in the background regardless of what's focused. Design anything time-based around the user actually having that screen open, or use Scheduled Reminder instead if it needs to fire at a specific time regardless of what's open.