Contact transfers between a new and old iPhone or Android phone are usually handled automatically by the operating system of the phone itself if the same email account is registered on both phones. It is also possible to transfer contacts between different phone operating systems with some additional steps.
Apple's iPhone and iOS operating system have the most straightforward process for transferring contacts, as any contacts registered on one iPhone are normally transferred to another iPhone from iCloud storage during the registration process for the new phone. It may be necessary to connect the new phone directly to a computer that has been synchronized with the old phone if the contact list has not been uploaded to iCloud, however.
Android phones do not have the option to transfer contacts from a synchronized computer without using third-party apps, but otherwise transfer contacts in a similar manner to iOS devices using the Gmail account of the owner of the new phone. The option to transfer contacts is located in the Accounts menu located in the Settings app of the phone. Synchronizing the contact list on the old phone through this interface allows the contact list to be uploaded automatically on the new phone when the owner signs in again. It is also possible to perform a similar transfer of contacts from an iPhone to an Android phone by choosing to sync contacts to a Gmail account rather than iCloud on the iPhone.