The smartphone contains magnets as well as components and radios that emit electromagnetic fields. All MagSafe accessories also contain magnets and the MagSafe Charger and MagSafe Duo Charger contain radios. Apple says that these magnets and electromagnetic fields might interfere with medical devices.
“Though all iPhone 12 models contain more magnets than prior iPhone models, they’re not expected to pose a greater risk of magnetic interference to medical devices than prior iPhone models,” according to the support document. “Medical devices such as implanted pacemakers and defibrillators might contain sensors that respond to magnets and radios when in close contact.”
To avoid any potential interactions with these devices, keep your iPhone and MagSafe accessories a safe distance away from your device (more than six inches apart or more than 12 inches apart if wirelessly charging).