Flutter 2.10 firebase - Sending notifications from device to device android and IOS

Question

I see many "similar" questions but none has clear or updated answers, especially for the new gen of flutter and firebase

I have a delivery app, where delivery man can update package to "delivered", once he update it I want to send notification to package owner that it was delivered.

what is the best way to gain the required? how to send notification from one device to another device ?

Im developing in flutter and firebase

All Tutorials I found talks about sending through firebase itself

Thanks in advance!