The domain name courier.push.apple.com is owned by Apple.
It is used by the Apple Push Notification Service (APNS) to alert Apple iOS and Apple macOS devices of notifications from apps, mail, etc.
You may see many connections to this domain name when looking at web traffic logs, such a proxy or firewall logs.
Further information on Apples push notifications at https://developer.apple.com/documentation/usernotifications#//apple_ref/doc/uid/TP40008194-CH8-SW1
I am one of the editors here at www.systemtek.co.uk I am a UK based technology professional, with an interest in computer security and telecoms.