The only 'special treatment' that happens is on Google/Apple servers which determine the priority of some of notifications.
How do I know? We're very involved with the XMPP protocol. The irony is that it is insanely hard to maintain an XMPP connection to a client on both Android / iOS, despite devices under both these OSs being an always connected XMPP clients.
Moxie has cited this previously with no proof, despite protocols like xmpp performing similarly on mobile networks.
The issue is that a long lived TCP connection consumes some valuable RAM and a port number on a valuable IP address. It costs carriers something for push notifications to work, so, they tend to aggressively close long lived TCP connections that aren't going to recognised push datacenters.
For that to work you need business relationships with the carriers of course, stable IP addresses, and enough users of your push service for it to be worthwhile.
IPv6 would fix it by eliminating IP scarcity and CGNAT, but most mobile networks aren't provisioning handsets with IPv6 addresses.
https://web.archive.org/web/20170708204409/www2015.wwwconfer...
Whether FCM uses these technologies, I don't know. I lack the hardware to easily inspect the mobile radio traffic on the necessary level (the best would be a DIY stingray based on one or two OsmocomBB-capable GSM phones, maybe combined with a Faraday cage to place one of them into, together with the smartphone.
They may not perform anticompetitive behaviour like bundling (as they have already been fined for in the EU over Android and Play Services).
I don't see the anti-competitive part there. There are good reasons around consolidating resident daemons and only using a single TCP connection with a special keep-alive configuration.
And from experience, it's perfectly possible to not use FCM, at the cost of a persistent notification that alerts the user about this, so they can get rid of abusive apps.
Maybe not exactly anti-competitive (but maybe): It requires you to have hundreds of megabytes of Google blobs with root permissions on your device, which some people don't like.
If course this pales in contrast to Apples actions.