Is there a standard understanding of what are push notifications? Are they the same for android and iOS? I’m not sure what this project is about!
But yes, it's essentially implemented the same across iOS/Android and browsers like Chrome, FF, Edge etc. There are services such as Amazon SNS or Google Firebase that provide a single interface which can talk to all the slightly different implementations.
In a nutshell the device registers to the server and receives an identifier token. Then a server can send messages to that identifier and the message will be displayed on the screen. Some providers allow for Icon, Audio and some Actions to also be sent as part of the message payload.
Exactly that. A device registers with the server and then there's a REST API or CLI binary that you can use to send it push notification messages.