Thanks for the hint but I am not entirely sure which icon you are referring to unfortunately.
Is it an icon provided by the application? Otherwise, this is an Android phone (LineageOS to be specific) and I am not sure there would be an Apple sharing icon on it.
Not all notifications go through FCM but all push notifications do as far as I’m aware - which is what the previous comment and the post title are talking about.
It is, in fact, worrying for privacy implications on the one hand and a real monopolizing factor on the other since if you wish to deliver an app which needs to implement such notifications you’re using Google’s service or constantly drain the user’s battery.
There’s UnifiedPush which tries to provide an open alternative but so far unfortunately still sees very little adoption.