WARNING YOU MUST USE ANDROID FOR THIS PART TO WORK.
FIREBASE FOR APPLE SET UP AT THE END IT’S ONE ENCRYPTION CERTIFICATE STORY AND THAT’S ANOTHER STORY! 
Hello,
Before tackling notifications, you have to understand what we are talking about and how it works.
You shouldn’t try to integrate this into a fully functional application, but simply manage to send one simple one.
BUT DON’T FORGET THE MANDATORY PREREQUISITE!
The application must be compiled and published on the STORE it is totally unavoidable. In INTERNAL TEST mode in Android or in TESTFLIGHT mode in Apple it is enough, but it is the minimum.
Why?
I will summarize the scheme of a notification, I note that it seems for everyone so logical that it is never done:
For example when you send a notification to a Token ID… In Firebase we use a request with this Token ID… And for the notification to reach its recipient, Google must know about its existence. And this happens through the application store of the smartphone. The Token ID is mainly generated in the application during the installation (it rarely changes afterwards, but I couldn’t define when and why. I know that it changes if you uninstall and reinstall the application).
Then, let’s imagine that Google keeps a directory that tells which application is installed by which Google AppStore account thanks to this token ID.
So if you don’t compile the application and you don’t install it through the store the token ID will never be known by Google and they won’t be able to make the link.
Then for subscriptions to topics or particular groups in the application, it is another story, but the logic remains the same.
IMPORTANT FOLLOW UP:
Now if the application is compiled and installed by the Store… And that in the application the Token ID has been generated and displayed on a blank page for example in the possible selection mode to be able to copy and paste it. Then, launch the application, display this page… The TOken is there!? If yes, copy it and close the application.
Then, go on POSTMAN.CO it is impossible to circumvent to carry out tests. (create an account it’s free)
Create a POST request
Enter this URL : https://fcm.googleapis.com/fcm/send
In Authorization :
Type: APIKey
Key: Authorization
Value : key=AAAAOb8HuMM:APA91bGAKdsKcMCbnDyGuFmF7UHpoLeiFb8nofTkKhF02T0bXWi5UZZMHCvNpBX9DMkeSm-GjHVLO_NOIxtaM5sZMv4E5Bo8Jxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx (your Firebase key)
Add to : header
In Body:
{
“priority”: “high”,
“to”: “YOUR TOKEN ID”,
“notification”:
{ “title”: “Does it work?”,
“body”: “We have a BUG!”
}
}
If the application is closed on the mobile… THE NOTIFICATION ARRIVES!
I’ll try to take more time to make complete explanations including Firebase setup and all the commands to test in Postman and then integrate in AppGyver.
Sorry for my English it is translated by DeepL