Just figured I'd add Apples own documentation as well

https://developer.apple.com/documentation/usernotifications/registering-your-app-with-apns

Apple Push Notification service (APNs) must know the address of a user’s device before it can send notifications to that device. This address takes the form of a device token unique to both the device and your app. At launch time, your app communicates with APNs and receives its device token, which you then forward to your provider server. Your server includes that token with any notifications it sends.