The standard operating procedure for using a magnetic mount with a phone is to affix a magnetic plate to the outside of the phone case; see the examples (below) from RAM and Bulletpoint.
Inspired by a customer comment @ amazon.com, I wanted to test a different approach -- use either a 65mm or 88mm magnetic mount from Arkon and a magnetic plate situated inside the phone case. But can a really strong magnet like this one harm the internal components of a phone ?
links:
https://www.bulletpointmountingsolutions.com/products/magnetic-phone-mount-holder
https://rammount.com/products/rap-300-1u
RAM Mounts RAM-B-348U Ball Adapter with 1/4"-20 Threaded Hole with B Size 1" Ball
https://rammount.com/products/ram-b-348u
ARKON Mounts 88mm Diameter Round Heavy-Duty Magnetic Base with 1/4”-20 Camera Mounting Bolt
https://www.amazon.com/ARKON-Diameter-Heavy-Duty-Magnetic-SP142088MAG/dp/B08N9SZTCZ/
ARKON Mounts - 65mm Diameter Round Heavy-Duty Magnetic Base Mount
https://www.amazon.com/ARKON-Diameter-Heavy-Duty-Magnetic-SP1420MAG25/dp/B088DB1KLL/
"I use a 2-piece Otterbox Defender on my android phone. The outer case is rubber, & the inner is a hard plastic shell, which snaps over the phone. I attached a large metal plate to the plastic shell, versus on the phone itself.
Unfortunately most magnetic-mount holders can't penetrate both layers, but the large magnet on this holder can! I used an adhesive Ram Mount as a base, added an extender arm, then inverted & attached the mag mount to the other end of the arm. EasyPeasy & my phone's secure!"