Apple published a safety alert concerning all models of the iPhone 12 (iPhone 12, iPhone 12 mini, iPhone 12 Pro, iPhone 12 Pro Max) for those users that might have a “medical device,” which is a generic way of saying: a pacemaker. These new phones include a ring of magnets that allow them to automatically latch on to MagSafe chargers for Apple iPhones. You can read the Apple alert here:
If you would prefer a much more technical explanation, see this abstract from the Heart Rhythm Journal,
This doesn’t appear to be significantly different from past recommendations by phone manufacturers to carry the devices away from implanted health devices. The new wrinkle seems to be the ring of magnets for MagSafe technology, combined with the NFC (Near Field Communications) capabilities of MagSafe.