I use a in iPhone 10 without a SIM card for my MM controller.  I connect to my daily cell phone (iPhone 11) via Bluetooth so I can have the Google Map display when flying.  This uses a lot of battery power so I am wondering if it's possible, and how, to have maps downloaded and available fo DJY Fly app to use instead so I don't have to tether.

Thanks,
Bob

The only way I know is open DJI Fly app when you have Wifi then open the map and find the zone where you want to fly , a portion of the map wil be downloaded as usual.

then don't exit the app , and travel to the zone where you want to fly your drone.  This is the only way to have a "offline map" .   When you close the app the map cache is cleared

Hope this help!