[ Preface: since all these use power that you might need to make an emergency call, power management is big. Make sure you are charged up at the last minute, monitor charge, and bring a "power bank" backup battery that has a power level display (that you check). For long outings, a solar recharger can be handy.

[iPhone] (only?)
• Set up Emergency SOS under Settings > Emergency SOS
• Through the above, or in Settings > Health > Medical ID, set up your emergency contacts, medical information, and set it up to be transmitted on the emergency call and available under emergency access from the lock screen
• Set up "location tracking" for your emergency contacts, so they can use "Find your friend" to try to locate your phone (in case you are out of it, or you lost your phone - hopefully that will be a clue to where you are if you don't show up).

• Navigation / Situational awareness apps:

[iPhone and Android]
• Location apps:
• Online maps: (what the heck - you might have signal). Google Earth, Apple Maps, Google Maps, AllTrails, GAIA GPS, etc.)
• App to display your GPS coordinates, and easily transmit by SMS text and email (if you have signal) your coordinates, Apple Maps link and Google Maps link by SMS or email: I use the free app "Find my GPS Coordinates" (available for iPhone and Android)
• Compass app: (iPhone includes one as default)
• Off-grid digital map: We should have one - the one I've been using seems to have gone quirky, so I don't know what to recommend at this point.
• Visual Orientation app: I like Peakfinder - before your trip, you can download digital elevation models of your region for offgrid use. In the field, you can then display a synthetic view you can match with your visual. I use it to get more accurate bearings than I can get with a magnetic compass, and it is a valuable check that I'm reading the compass correctly, too. You can get a feel for it on the free website - the iPhone/Android apps for off-grid use are about $5 and linked from the website. See here: https://www.peakfinder.org/?lat=34.1091&lng=-119.0645&azi=157&zoom=5&ele=429&cfg=r&date=2021-01-16T22:38Z
• Astronomy app: Mine's actually on a PalmPilot, but by day solar compass is more accurate than magnetic, and by night, stellar orientation even more so.
• Weather app: ( I use the free iPhone "Weather Radar" app a lot - even for neighborhood walks).
• Photos/video of my route: (particularly the photo of the turnoff we overshot by a mile once despite GPS ...) - no app required, but I use Google Maps satellite imagery and Google Maps streetview imagery as well as online photos from hikers as prep. I normally print that as hardcopy and carry in my backpack (not battery dependent), but it can go in the phone, too.

• Skills apps

• First aid books/apps; On my iPhone, I have "First Aid" - by the American Red Cross, I think.
• Survival books: (better late than never, and even if you think you remember, you may want to double-check your memory). Also, once they are on your phone, you can read them in odd moments before the emergency - like while waiting in line for your COVID vaccination ...
• Knots: "Animated Knots" app - https://www.animatedknots.com/shop
• .pdf manuals for any fancy gear I'm taking (camera, ham radio, GoPro, and yes, your smartphone) [I bring hardcopy, too.]

I'm sure I've overlooking whole categories of things, but this is what I have after a quick look in the "hiking" folder of my iPhone (21 apps currently), and skimming some of my other 100+ apps to see what might be useful ...