Trail Sense uses phone sensors to help user in survival.
OsmAnd can download maps for offline use. (Tutorial would be good on how to set it up for survival - map coloring, highlighting POI)
F-Droid should be also recommended, because it has functionality to share apps without Internet connection.
Briar is a messaging app, which doesn't require internet.
I've added Briar to the Apps chapter as I am interested in its functionality. From the looks of it, in a complete network outage, it would be limited to only 10 metres as that's the range offered by Bluetooth.
Trail Sense uses phone sensors to help user in survival. OsmAnd can download maps for offline use. (Tutorial would be good on how to set it up for survival - map coloring, highlighting POI) F-Droid should be also recommended, because it has functionality to share apps without Internet connection. Briar is a messaging app, which doesn't require internet.