How are Apple devices maintaining persistent connections with Apple servers for things such as APN, FindMy network, etc

Apple devices such as iPhones and Macs are able to receive near real-time notifications from the APN system, as well as frequently checking in with the FindMy network, and more. I am wondering how these devices are able to maintain persistent connections with Apple servers. I assume that Apple devices would not use something like polling, since that would likely eat up battery and waste cellular data. I'm wondering if Apple uses something like WebSockets , or maybe their own proprietary system?