Google runs the notification delivery on Android. It is part of of Firebase, which is present in almost every app on your phone

Firebase has analytics built in, on by default. Therefore Google gets to know how users use all other apps, so they can release their own version or just to improve their AI.

I think it is free, but only till a certain number of notifications. (Not sure, I am not an android dev)

https://firebase.google.com/docs/cloud-messaging/