Trust me that you want your phone in a hydration pack and not where it can get damaged on your bike, and that you want a hydration pack regardless so you can have water, tools, and appropriate carry along stuff.
I'm fairly in tune with what works well for the sport. I've been doing the riding since before real mountain bikes existed, and infrastructure and club management for the sport for the past 14 years.
Your standard Apple buds allow you to know what's around you and some foam covers available at Radio Shack and other places do a lot to keep them in.
IMBA has a partnership with mtbproject.com for mapping and in some areas that's your best source for a map because the maps are curated by the same people who make your trails possible. Your IMBA membership and especially selecting your local chapter should also be considered important accessories. You've chosen a great sport that requires the work that volunteers and IMBA do.
For mapping and tracking apps the ones from Abvio are great via the way they can be bought as for MTB, running etc... but they track many sports and have an option to be tied to Apple's Health.
One more great mapping app is Gaia GPS. Its price has grown because it has such great capabilities. With it you can download maps for off line use so you have superb performance without relying on the cel networks.
Rokform seems to be the most popular phone and bike holder among my associates but not for real trail riding where your chances for a crash are there.
My experience with 14 years of trail steward, managing a bike park, and managing trails at a ski area say you look in the mirror, raise you hand, and call yourself an ***** and a jerk if you wear the sort of earphones that isolate the noise around you. You risk not knowing if something is wrong with your bike, risk injury because others may not be aware you don't hear them, and not being aware of others when you're in shared public space is rude (putting it mildly).
Have lots of fun with your new sport and phone! Look for the MTB club or IMBA chapter near you for the best possible advice and accelerating the rate that you have a lot of fun and build skills.
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