One of the strategies I use when I’m looking for a new mountain bike ride is to scour Strava for the route, then download the GPX file. Once I have that GPX file I can upload it to the Gaia GPS web site.
Inside Gaia GPS on my phone (which is iOS and Android compatible) I can download “offline” maps, which means I can have a fully usable map without data service when I’m out in the wilderness on an unfamiliar trail.
I don’t have my phone out for the entire ride, I just pull it out like I would any map, when I need guidance.
If the route you’re looking for is more well-known and has been added to MTBproject.com, then you can skip all of this and use the MTBproject app, which has offline mapping built in. The standards for getting a ride on MTBproject site are pretty high, so a lot of trails are missing.