To add to what has been said already (and to add more to the confusion maybe):
If you did not download the map of the proposed hiking area, and you had a good look to the area in your app just before you start your hike, and you do NOT close the app and NOT switch off your phone completely, the map might still be available even if you leave the area of signal out into the backcountry, because the map data might still sit in the smartphone cache memory.
This effect, while doing good at the moment, might lead you to false assumptions of having the whole data available. But no, the cached data will get lost either when closing the app, or at least when swithing off the phone.