How do push notifications work in ios (more in the table of contents)?

Question

iOS

The iphone has several applications installed (mail, messengers, and others where push notifications are involved).
It happens that some push notifications do not come. For example, in viber: I find out that a person called me only after a couple of hours, when I manually launch the application myself. And it happens that a push notification about a call or message arrives immediately.
The same thing happens with mail apps.
1) What could be the reasons for this?
2) Could this be due to the fact that too many applications are installed on the smartphone?
PS Official apple sources about how push notifications work have already been read.
PPS There are no problems with the network.

Answer 1

If you have read Apple sources, you should know that push notification delivery is not guaranteed. In most cases, of course, they come, but nonetheless.
The fact that you are not receiving notifications from Viber may be a problem with the Viber infrastructure.