Your smartphone will not be damaged by a magnetic car holder. Previously, magnetic sequencing was used to store memory in/on hard drive devices, but this is not so for the modern chip technology, which is made of materials that are non-magnetic. As a result, magnets will not impact their functioning.

Does the magnet impact the memory? There are no longer Hard disk drives (HDD) in phones that store data using magnetic fields. Phones actually use storage chips, which are a kind of flash memory and magnets have no impact on this.

Some forums mention that iPods might be impacted by magnets, but this is only true for the iPod Classic which uses HDD. Flash memory is used in other mobile devices and should not be a concern.