I know this has been asked before but not in a couple of years, looking at search results, and the app landscape changes quickly.
Personally I'm looking for a free Android app but will consider paid if they're demonstrably safer in some way.
Don't hold back on tips for iPhone users either though as it's nice to have all the info in one place.
If there's going to be a phone/data signal, you can share your location in Google Maps
Google maps app (and Apple maps) can share your current location with a couple of clicks, used this a couple of times when I've been stranded after a mechanical... or are you after something that will continuously broadcast your location (in case you're incapacitated)? In which case Apple has "find friends" which can be always on. Sure Android/google has similar.
Nothing will be that much help if you've no phone signal though (although Find Friends will display last known position)
Some GPS devices (like my Garmin 820, though I've never used the feature) will send a text/email message with a location to a pre-determined contact (or contacts) if it detects a crash and you don't manually cancel it within a set time.
Find My Friends, or, more easily, Google Maps, as above. Always share my location with someone before heading out for a ride.
I'd have thought us MTBers always going to places with no signal, definitely need last-known-location sharing. Ideally with a bit of a track log as well to give an idea of direction.
I use Buddy Beacon on Viewranger with the wife. Gives live tracking while you have a signal, time of last broadcast with speed and direction. Handy for what we do. You both need the app but its free.