Under normal operation the release bearing doesn't move very far forward. So it won't slip off the end.
The input shaft (comes out of the gearbox) is fixed, and slides into the clutch plate.
The clutch cover has a large spring in it that puts pressure on the plate, pushing it against the fly wheel.
The clutch cover is bolted directly to the flywheel, over the clutch plate.
So the plate is directly connected to the g/box and cover is directly connected to the engine.
The only thing connecting the g/box to the engine then is the pressure from the clutch cover against the clutch plate.
The release bearing works against the clutch cover spring, removing the pressure on the plate, which seperates the connection between the g/box and engine.